Is he sinning if he sees an evil action and does not denounce it?
Question

                Will I be called to account for every evil I see? If I see, for example, people I do not know slandering, or fighting, or if I see someone listening to music, or hear something wrong about the Sunnah like a weak Hadeeth or anything like this. Will I be brought to account in the Day of Resurrection if I do not advise them?.
Praise be to Allah.Firstly:
Enjoining what is good and forbidding what is evil is one of the greatest principles of Islam. Some scholars regard it as a pillar of Islam. What every Muslim must do is enjoin what is good and forbid what is evil. This does not apply only to some Muslims and not others, rather it is obligatory upon every Muslim who knows anything of the religion of Allaah to convey it, whether it is a command or a prohibition.  
Allaah says (interpretation of the meaning): 
“You (true believers in Islamic Monotheism, and real followers of Prophet Muhammad صلى الله عليه وسلم and his Sunnah) are the best of peoples ever raised up for mankind; you enjoin Al-Ma‘roof (i.e. Islamic Monotheism and all that Islam has ordained) and forbid Al‑Munkar (polytheism, disbelief and all that Islam has forbidden)”
[Aal ‘Imraan 3:110] 
“Let there arise out of you a group of people inviting to all that is good (Islam), enjoining Al-Ma‘roof (i.e. Islamic Monotheism and all that Islam orders one to do) and forbidding Al-Munkar (polytheism and disbelief and all that Islam has forbidden). And it is they who are the successful”
[Aal ‘Imraan 3:104]
“Those among the Children of Israel who disbelieved were cursed by the tongue of Dawood (David) and ‘Eesa (Jesus), son of Maryam (Mary). That was because they disobeyed (Allaah and the Messengers) and were ever transgressing beyond bounds.
79. They used not to forbid one another from Al-Munkar (wrong, evildoing, sins, polytheism, disbelief) which they committed. Vile indeed was what they used to do”
[al-Maa’idah 5:78, 79]
Ibn ‘Atiyah (may Allaah have mercy on him) said: 
There is consensus that forbidding evil is obligatory for the one who is able to do it, and is safe from harm to himself or the Muslims. But if he fears (harm) then he may denounce it in his heart and shun the doer of evil and not mix with him. 
See: Tafseer al-Qurtubi (6/253). 
Secondly: 
In order for this principle to be implemented there are conditions that must be met, most of which have to do with the person who is denouncing it, and some of which have to do with the principle itself. 
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him) said: 
The first condition is that he should know the Islamic ruling concerning that which he is enjoining or forbidding, so he should only enjoin that which he knows that sharee’ah enjoins, and he should only forbid that which he knows sharee’ah forbids, and he should not rely on his taste or customs with regard to that. 
The second condition is that he should know the situation of the person addressed: is he one who should be enjoined or forbidden, or not? If he sees a person who he is not sure whether he is accountable or not, he should not enjoin anything upon him until he finds out. 
The third condition is that he should know about the person who appears to be accountable: has he done the action he wants to enjoin, or not? 
If he sees someone enter the mosque and sit down, and he is not sure whether he did the two rak’ahs or not, he should not denounce him or tell him to do them, rather he should find out more.  
The fourth condition is that he should be able to enjoin what is good and forbid what is bad, without bringing harm upon himself. If it will bring harm upon him, then he does not have to do it, but if he is patient and does it, that is better, because all duties are subject to the condition that one be able to do them, because Allaah says (interpretation of the meaning):  
“So keep your duty to Allaah and fear Him as much as you can”
[al-Taghaabun 64:16]
“Allaah burdens not a person beyond his scope”
[al-Baqarah 2:286]
The fifth condition is that enjoining what is good and forbidding what is evil should not result in any evil greater than keeping quiet. If that will result then he does not have to do it, rather it is not permissible for him to enjoin what is good and forbid what is evil. 
Majmoo’ Fataawa al-Shaykh al-‘Uthaymeen (8/652-654). 
Thirdly: 
Once you know that enjoining what is good and forbidding what is evil are two of the duties of sharee’ah, and the shar’i conditions for doing it are fulfilled, then you must understand that if you see evil being committed, then not advising the one who is doing it means that you are sinning, unless someone else denounces it, because it is an individual obligation (fard ‘ayn) upon you if you see evil being committed and there is no one else to advise the evildoer except you, but if there is someone else who undertakes to advise the evildoer, then it becomes  a communal obligation (fard kifaayah) and you are not sinning if someone else fulfils the duty. 
Ibn Qudaamah (may Allaah have mercy on him) said: 
Some people say that enjoining good and forbidding evil can only be done with the permission of a ruler or governor, but they do not allow ordinary individuals to do this, but this is wrong because the verses and reports indicate that everyone who sees an evil and keeps quiet is sinning, so there is no need for the permission of the ruler. 
Mukhtasar Minhaaj al-Qaasideen (p. 124). 
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) said: 
Enjoining what is good and forbidding what is evil may be an individual obligation, in the case of one who sees an evil when there is no one else to denounce it, and he is able to denounce it. In that case he is individually obliged to denounce it because there is a great deal of evidence to that effect, among the clearest of which is the hadeeth in which the Prophet (peace and blessings of Allaah be upon him) said: “Whoever among you sees an evil action, then let him change it with his hand [by taking action]; if he cannot, then with his tongue [by speaking out]; and if he cannot, then with his heart – and that is the weakest of faith.” Narrated by Muslim in his Saheeh. 
Fataawa al-Shaykh Ibn Baaz (3/212). 
Fourthly: 
It makes no difference whether these evils take place in the street, the home or the workplace; if a Muslim is able to denounce them without that leading to harm, then he has no excuse for not denouncing them.
 Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) said: 
The believing men and women enjoin what is good and forbid what is evil, and the believer does not keep quiet. If he sees his brother committing an evil, he denounces him. Similarly, if he sees his sister, paternal aunt, maternal aunt or anyone else committing an evil action, he tells them not to do that. If he sees his brother in faith or his sister in faith falling short in some duty, he denounces him for that, and enjoins him to do what is good. All of that is to be done with kindness and wisdom, and good manners. 
If the believer sees one of his brothers in faith being lazy in praying, or engaging in backbiting or gossip, or smoking or drinking, or disobeying one or both of his parents, or severing the ties of kinship, he denounces him in kind words and with good manners, not with hateful words and harshness, and he explains to him that it is not permissible for him to do this thing. 
All of these evils must be denounced by every believing man and woman and every righteous person, by husbands and wives, brothers, relatives, neighbours, friends and others. They must all do that, as Allaah says, describing the believing men and women (interpretation of the meaning):
“they enjoin (on the people) Al‑Ma‘roof (i.e. Islamic Monotheism and all that Islam orders one to do), and forbid (people) from Al‑Munkar (i.e. polytheism and disbelief of all kinds, and all that Islam has forbidden)”
[al-Tawbah 9:71]
And the Prophet (peace and blessings of Allaah be upon him) said: “If the people see an evil and do not change it, soon Allaah will punish them all.” 
And he (peace and blessings of Allaah be upon him) said: “Whoever among you sees an evil action, then let him change it with his hand [by taking action]; if he cannot, then with his tongue [by speaking out]; and if he cannot, then with his heart – and that is the weakest of faith.” 
This is general in meaning and applies to all evil actions, whether they are on the road, in the home, in the mosque, on an airplane, on a train, in a car or anywhere else. This applies to both men and women. A woman can speak and a man can speak, enjoining what is good and forbidding what is evil, because this is in the interests of all and concerns the salvation of all. 
It is not permissible to keep quiet about that to avoid upsetting the husband, brother or anyone else. But it should be done with good manners and kind words, not with violence and harshness, paying attention to suitable times. Some people may not be open to advice at some times, but at other times they may be willing to accept it. The believing man and woman pay attention to the appropriate times for denouncing evil and enjoining good, and they should not despair and think that the one who does not accept it today will not accept it tomorrow. The believing man should not despair and the believing woman should not despair, rather they should carry on denouncing evil and enjoining what is good with sincerity towards Allaah and His slaves, and thinking positively of Allaah and hoping for the reward that is with Allaah. 
Fataawa al-Shaykh Ibn Baaz (4/50). 
Fifthly: 
Does that mean that he has to denounce everyone who shaves his beard, or everyone who lets his lower garment hang too low, or every swearing person whom he hears? Does he have to pick up every paper on the street on which there is the name of Allaah? Does he have to stand up and speak to every khateeb whom he hears quoting a weak or fabricated hadeeth?  
The answer to all of that – and similar cases – is no, he does not have to do that, because no one who calls people to Allaah can do all of that, especially since most of those whom he sees doing evil actions may know the ruling of Allaah on their actions, and proof has been established against them. If Islam required him to do that he would spend his whole life standing with those sinners and advising them as the expense of his own interests and livelihood, and this is something that can never be prescribed by sharee’ah. 
The scholars of the Standing Committee said: 
In order to avoid sin and blame, it is sufficient for you to advise people not to use papers on which the name of Allaah appears, because that is showing disrespect, and to warn them against throwing them into garbage pails, in the streets, in their neighbourhoods and so on. You are not obliged to put yourself through hardship by devoting all your time to collecting the papers that are scattered in the streets and so on, rather you may pick up whatever you can without causing yourself hardship. 
Shaykh ‘Abd al-‘Azeez ibn Baaz, Shaykh ‘Abd al-Razzaaq ‘Afeefi, Shaykh ‘Abd-Allaah ibn Ghadyaan, Shaykh ‘Abd-Allaah ibn Qa’ood. 
Fataawa al-Lajnah al-Daa’imah (4/73-75). 
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allaah have mercy on him) said: 
Enjoining what is good and forbidding what is evil is a communal obligation (fard kifaayah); if sufficient people undertake it, it is waived for the rest, but if not enough people undertake it, then all the people are obliged to enjoin what is good and forbid what is evil. But it is essential that it be done with wisdom, kindness and gentleness, because Allaah sent Moosa and Haroon to Pharaoh and said (interpretation of the meaning): 
“And speak to him mildly, perhaps he may accept admonition or fear (Allaah)”
[Ta-ha 20:44]. 
As for violence, whether in word or deed, this is contrary to wisdom and is contrary to what Allaah has enjoined. 
But sometimes a person may be faced with something and say “This is a well known evil,” such as shaving the beard for example, which everyone knows is haraam – especially those who live in this land (Saudi Arabia) – and he says: if I were to stop every time I see someone with a bare face – and how many of them there are – and tell him not to do this, then I would miss out on a lot of good. In this case, perhaps we should say that the duty of denouncing is waived, because that would cause him to miss out on a lot of good. But if you happen to meet this man in a shop, restaurant or café, in that case it would be good to remind him to fear Allaah and say: This is something haraam, and if you persist in a minor sin then in your case it will become a major sin – and say something appropriate.
Liqaa’aat al-Baab il-Maftooh (110/ question no. 5). 
We feel very sorry for the Muslims who disobey their Lord in their manner of dressing, behaviour, actions and words. We encourage everyone who is able to convey the pure message of Islam to these people to do so. When a Muslims hears or sees an evil that he is able to denounce, he should do so, but he does not have to do that with regard to every evil he sees or hears. Undoubtedly evil things vary in their severity. And some of them involve transgression against other people whilst others affect only the person who does them. In cases where the evil is particularly serious or offensive, denouncing them is a more urgent duty for the Muslims than evils that are less serious. Whoever hears someone reviling Allaah or Islam, the sin and offence involved is not like that in a case where he hears someone reviling some other person. 
If the sin transgresses against others, then denouncing it is more urgent then an evil which affects only the person who does it. What matters is that the Muslim’s heart should remain aware and denounce the evil even if he is unable to take action or speak out against it. But the one who can change the evil by taking action or speaking out against it should not fail to do so.  
Shaykh ‘Abd al-‘Azeez ibn Baaz (may Allaah have mercy on him) said: 
Denouncing evil in one's heart is obligatory for everyone, because everyone can do it. What is means is hating the evil and keeping away from those who do it when one is unable to denounce it by taking action or speaking out, because Allaah says (interpretation of the meaning): 
“And when you (Muhammad صلى الله عليه وسلم) see those who engage in a false conversation about Our Verses (of the Qur’aan) by mocking at them, stay away from them till they turn to another topic. And if Shaytaan (Satan) causes you to forget, then after the remembrance sit not you in the company of those people who are the Zaalimoon (polytheists and wrongdoers)”
[al-An’aam 6:68]
And Allaah says in Soorat al-Nisa’ (interpretation of the meaning):  
“And it has already been revealed to you in the Book (this Qur’aan) that when you hear the Verses of Allaah being denied and mocked at, then sit not with them, until they engage in a talk other than that; (but if you stayed with them) certainly in that case you would be like them”
[al-Nisa’ 4:140]
And He says (interpretation of the meaning):  
“And those who do not bear witness to falsehood, and if they pass by some evil play or evil talk, they pass by it with dignity”
[al-Furqan 25:72]
i.e., they do not witness it or attend such gatherings. 
Fataawa al-Shaykh Ibn Baaz (3/212, 213). 
And Allaah knows best.