Is Failing to fulfil One Right of a Muslim a Sin?
Question

We know the Hadith of the Messenger (blessings and peace of Allah be upon him) about the rights of one Muslim over another. 
My question is: Will we be sinning if we do not fulfil one of these rights towards our Muslim brother? i.e., will we incur a burden of sin thereby?
Praise be to Allah.Will you be sinning if you fail to fulfil one of the rights of another Muslim?
The rights that one Muslim has over another are many; some of which are individual obligations that are required of each person, and if he fails to do them he is sinning. Others are communal obligations; if some people do them, the burden of sin is waived for the rest. And some are encouraged but not obligatory, so the Muslim is not sinning if he does not do them. 
How many rights does a Muslim have over another Muslim?
Al-Bukhari (1240) and Muslim (2162) narrated that Abu Hurayrah (may Allah be pleased with him) said: I heard the Messenger of Allah (blessings and peace of Allah be upon him) say: “The rights of one Muslim over another are five: returning the greeting of Salam, visiting the sick, attending funerals, accepting invitations, and saying Yarhamuk Allah (may Allah have mercy on you) to one who sneezes.” 
And Muslim (2162) narrated from Abu Hurayrah (may Allah be pleased with him) that the Messenger of Allah (blessings and peace of Allah be upon him) said: “The rights of one Muslim over another are six.” It was said: What are they, O Messenger of Allah? He said: “If you meet him, greet him with Salam; if he invites you, accept the invitation; if he asks for advice, give him sincere advice; if he sneezes and praises Allah, say Yarhamuk Allah (may Allah have mercy on you); if he falls sick, visit him; and if he dies, attend his funeral.” 
Ash-Shawkani (may Allah have mercy on him) said: “What is meant by the words “The rights of the Muslim” is that they should not be omitted and doing them is either obligatory, or encouraged to such an extent that it is very similar to being obligatory and should not be omitted. The word “right” (Haqq) may be used in the sense of obligatory, as was mentioned by Ibn Al-`Arabi.” (Nayl Al-Awtar, 4/21) 
·         Returning the greeting of Salam is obligatory if the greeting is given to one person. If it is given to a group, then it is obligatory upon the group (Fard Kifayah or communal obligation; if one of the group returns the greeting, the obligation has been met). With regard to initiating the greeting, the basic principle is that it is encouraged. 
It says in Al-Mawsu`ah Al-Fiqhiyyah (11/314): 
“Initiating the greeting is Sunnah Mu’akkadah (a confirmed Sunnah) because the Prophet (blessings and peace of Allah be upon him) said: “Spread the greeting of Salam amongst yourselves.” It is obligatory to return the greeting if it is given to one person. If the greeting is given to a group, then in their case responding is a Fard Kifayah (communal obligation); if one of them responds the sin is waived from the others, but if they all respond, they have all done what is required, Whether they respond all together or one after another. If they all refrain from responding, then they are all sinning because of the report which says: “The rights of one Muslim over another are five: returning (the greeting) of Salam…” 
·         Visiting the sick it is a communal obligation . 
Shaykh Ibn ‘Uthaymin (may Allah have mercy on him) said: “Visiting the sick is a Fard Kifayah (communal obligation).” (Majmu` Fatawa wa Rasa’il Ibn `Uthaymin, 13/1085)
·         Attending funerals is also a Fard Kifayah (communal obligation). 
·         With regard to accepting invitations , if the invitation is to a wedding feast, then the majority of scholars are of the view that it is obligatory to accept unless there is a legitimate Shar‘i reason not to do so. If it is for something other than a wedding feast, the majority are of the view that it is encouraged. But there are conditions for accepting invitations in general terms. 
·         With regard to saying Yarhamuk Allah (May Allah have mercy on you) to one who sneezes, there is a difference of opinion concerning the ruling. 
It says in Al-Mawsu`ah Al-Fiqhiyyah, 4/22: 
“This saying Yarhamuk Allah (May Allah have mercy on you) is encouraged according to the Shafa`is. 
According to the Hanbalis and the Hanafis, it is obligatory. 
The Malikis said – and it is also an opinion among the Hanbalis – that it is a communal obligation. It was narrated from Al-Bayan that the stronger view is that it is an individual obligation (Fard ‘Ayn), because of the Hadith: “It is the duty of every Muslim who hears him (the one who sneezes) to say: Yarhamuk Allah (may Allah have mercy on you).
The more correct opinion is that it is obligatory on the one who hears the sneezer praise Allah (by saying “Al-hamdu Lillah”), because of the report narrated by Al-Bukhari (6223) from Abu Hurayrah (may Allah be pleased with him) from the Prophet (blessings and peace of Allah be upon him) who said: “Allah likes the act of sneezing and dislikes the act of yawning, so if any one of you sneezes and praises Allah (says ‘Al-hamdu Lillah’), it is a duty on every Muslim who hears him to say to him, ‘Yarhamuk Allah (may Allah have mercy on you).’” 
Ibn Al-Qayyim (may Allah have mercy on him) said: 
“We have quoted above the Hadith of Abu Hurayrah, in which it says: “if any one of you sneezes and praises Allah (says ‘Al-hamdu Lillah’), it is a duty (Haqq) on every Muslim who hears him to say to him, ‘Yarhamuk Allah (may Allah have mercy on you).”. At-Tirmidhi included the Hadith of Anas under the heading: Chapter on what was narrated about it being obligatory to say Yarhamuk Allah (may Allah have mercy on you) when one who sneezes says Al-hamdu Lillah (Praise be to Allah). This indicates that it is obligatory in his view, and this is the correct view, because of the Hadiths that clearly indicate that it is obligatory and there was nothing to contradict that, and Allah knows best. 
One of them is the Hadith of Abu Hurayrah mentioned above, and another is his other Hadith, “There are five (rights) that the Muslim has over his brother,” which is also mentioned above. Another is the Hadith of Salim ibn ‘Ubayd, in which it says: “Let those who are with him say to him: Yarhamuk Allah (may Allah have mercy on you).” And another is the report narrated by At-Tirmidhi from ‘Ali who said: The Messenger of Allah (blessings and peace of Allah be upon him) said: “The Muslim has six (rights) over his fellow-Muslim: he should greet him with Salam when he meets him; he should respond when he invites him; he should say Yarhamuk Allah (may Allah have mercy on you) if he sneezes; he should visit him if he falls sick; he should attend his funeral if he dies; and he should love for him what he loves for himself.” He (At-Tirmidhi) said: This is a sound Hadith. It was also narrated via another Isnad (chain of narration) from the Prophet (blessings and peace of Allah be upon him). Some of the scholars spoke negatively about Al-Harith Al-A‘war (one of the narrators). In the same chapter it is also narrated from Abu Hurayrah, Abu Ayyub, Al-Bara’ and Abu Mas‘ud. And another of these Hadiths is that which was narrated by At-Tirmidhi from Abu Ayyub, according to which the Messenger of Allah (blessings and peace of Allah be upon him) said: “If one of you sneezes, let him say Al-hamdu Lillah (praise be to Allah); and let him say ‘ala kulli hal (in all circumstances). And let the one who responds to him say: Yarhamuk Allah (may Allah have mercy on you). And let him say: Yahdikum Allahu wa yuslih balakum (May Allah guide you and set your affairs straight). 
There are four kinds of proof in the Hadith quoted above (“if any one of you sneezes and praises Allah (says ‘Al-hamdu Lillah’), it is a duty (Haqq) on every Muslim who hears him to say to him, ‘Yarhamuk Allah (may Allah have mercy on you)”):
1.      there is a clear statement that it is obligatory to say Yarhamuk Allah (may Allah have mercy on you), which cannot be interpreted in any other way;
2.      it is made obligatory by use of the word Haqq (translated above as duty);
3.      it is made obligatory by use of the word ‘Ala (on). This word clearly means that it is obligatory
4.      it is enjoined. There can be no doubt that there are many duties that are proven to be obligatory on the basis of less evidence than this. And Allah knows best.” (Hashiyat Ibn Al-Qayyim ‘ala Sunan Abu Dawud, 13/259) 
He (may Allah have mercy on him) also said: 
“The apparent meaning of the Hadith mentioned above is that saying Yarhamuk Allah is an individual obligation upon everyone who hears the one who sneezed say Al-hamdu Lillah; it is not acceptable for just one of them to say it. This is one of the two scholarly opinions, which was favoured by the Malikis Ibn Abu Zayd and Abu Bakr ibn Al-‘Arabi, and it cannot be otherwise.” (Zad Al-Ma`ad, 2/437) 
·         With regard to giving him advice if he asks for it, it is most likely that offering advice is a communal obligation. 
Ibn Muflih (may Allah have mercy on him) said: 
“The apparent meaning of the words of Ahmad and our companions is that it is obligatory to offer advice to the Muslim , even if he does not ask for it, as is the apparent meaning of the reports.” (Al-Adab Ash-Shar‘iyyah by Ibn Muflih, 1/307) 
Al-Mullah `Ali Al-Qari (may Allah have mercy on him) said: 
“If he asks you for advice” means if he asks you for advice, then give it to him; it is obligatory. It is also obligatory to give advice even if he did not ask for it.” (Mirqat Al-Mafatih, 5/213) 
Ibn Hajar (may Allah have mercy on him) said: 
“It is clear that what is meant by “duty” (Haqq) here is that it is obligatory. This is different from the words of Ibn Battal who said that what is meant is the duty of respect and companionship. It seems that what is meant here is that it is a communal obligation.” (Fath Al-Bari, 3/113) 
For more details, please see the following answers: 21878 and 65649
And Allah knows best.