Necessity of refraining from discussion of the disagreements that occurred among the Companions of the Prophet (blessings and peace of Allah be upon him)
Question

Is it true that ‘Ali ibn Abi Taalib and ‘Uthmaan (may Allah be pleased with him) were killed by some Muslim people? Can you explain what the problem was that arose among the Sahaabah during that period and tell me of some books that deal with this topic?.
Praise be to Allah.It should be understood – may Allah protect us and you from confusion and specious arguments, both obvious and subtle – that the Companions of Muhammad (blessings and peace of Allah be upon him) were the best companions of the best Prophet, especially the four Rightly Guided Caliphs. 
Allah says (interpretation of the meaning):
“And the first to embrace Islam of the Muhajiroon (those who migrated from Makkah to Al-Madinah) and the Ansar (the citizens of Al-Madinah who helped and gave aid to the Muhajiroon) and also those who followed them exactly (in Faith). Allah is well-pleased with them as they are well-pleased with Him” [at-Tawbah 9:100]. 
Ibn Katheer (may Allah have mercy on him) said: Here Allah the Almighty tells us that He is pleased with the first to embrace Islam of the Muhaajireen and Ansaar, and those who followed them in truth. So woe to those who hate them or revile them, or who hate some of them and revile some of them. End quote. Tafseer Ibn Katheer, 4/203 
Allah, may He be exalted, says concerning the Muhaajireen (interpretation of the meaning):
“(And there is also a share in this booty) for the poor emigrants, who were expelled from their homes and their property, seeking Bounties from Allah and to please Him. And helping Allah (i.e. helping His religion) and His Messenger (Muhammad (peace and blessings of Allah be upon him)). Such are indeed the truthful (to what they say)” [al-Hashr 59:8].
And Allah says concerning the Ansaar (interpretation of the meaning):
“And those who, before them, had homes (in Al-Madinah) and had adopted the Faith, love those who emigrate to them, and have no jealousy in their breasts for that which they have been given (from the booty of Banee An-Nadeer), and give them (emigrants) preference over themselves, even though they were in need of that” [al-Hashr 59:9]. 
And Allah says concerning the believers who come after them (interpretation of the meaning):
“And those who came after them say: Our Lord! Forgive us and our brethren who have preceded us in Faith, and put not in our hearts any hatred against those who have believed. Our Lord! You are indeed full of kindness, Most Merciful” [al-Hashr 59:10]. 
With regard to what happened among the Sahaabah of differences of opinion and fighting, we should refrain from discussing it, whilst believing that they are the best of the ummah and loving them and approving of them. This opinion has been repeatedly stated by Ahl as-Sunnah wa’l-Jamaa‘ah. 
‘Umar ibn ‘Abd al-‘Azeez (may Allah have mercy on him) was asked about ‘Ali and ‘Uthmaan, and the battles of the Camel and Siffeen, and what happened among them. He said: That is blood that Allah caused me to have no hand in shedding, and I do not want to dip my tongue in it (by talking about it). at-Tabaqaat al-Kubra, 5/394 
A man asked Imam Ahmad ibn Hanbal about what happened between ‘Ali and Mu‘aawiyah, and he turned away from him. It was said to him: O Abu ‘Abdullah, he is a man from Banu Haashim. He turned to him and said: Recite: “That was a nation who has passed away. They shall receive the reward of what they earned and you of what you earn. And you will not be asked of what they used to do” [al-Baqarah 2:134]. 
Manaaqib al-Imam Ahmad by Ibn al-Jawzi, p. 126 
Imam Ahmad also said, after it was said to him: What do you say about what happened between ‘Ali and Mu‘aawiyah? He said: I do not say anything about them except what is best. 
Manaaqib al-Imam Ahmad by Ibn al-Jawzi, p. 164 
Al-Maymooni said: Ahmad ibn Hanbal said to me: O Abu’l-Hasan, if you see a man saying anything bad about any one of the Sahaabah, then be suspicious about his Islam. Al-Fadl ibn Ziyaad said: I heard Abu ‘Abdullah being asked about a man who criticized Mu‘aawiyah and ‘Amr ibn al-‘Aas: can he be called a Raafidi? He said: He did not the audacity to criticise them except because he has some evil hidden in his heart.  No one criticises any one of the Sahaabah but he is concealing some evil in his heart. End quote. "Al-Bidaayah wa’n-Nihaayah, 8/139 "
Abu Zar‘ah ar-Raazi said: 
If you see a man criticising any of the Companions of the Messenger of Allah (blessings and peace of Allah be upon him), then know that he is a heretic, because in our view the Messenger (blessings and peace of Allah be upon him) is true and the Qur’aan is true. Rather this Qur’aan and the Sunnah were conveyed to us by the Companions of the Messenger of Allah (blessings and peace of Allah be upon him). They only want to undermine the status of those who conveyed the Qur’aan (and the Sunnah) in order to undermine the validity of the Qur’aan and Sunnah. But it is more fitting that they be criticised, and they are heretics. End quote. "Al-Kifaayah fi ‘Ilm ar-Riwaayah, p. 49 "
Al-Qurtubi said: 
It is not permissible to attribute any deliberate mistake to any of the Sahaabah, because they based all their actions on what they believed was right and proper, and their intention was to Allah, may He be glorified and exalted. They are all examples for us to follow and Allah has instructed us to  refrain from discussing the disagreements that arose among them, and we should only mention them in the best way, because of the sanctity of their being Companions of the Prophet (blessings and peace of Allah be upon him), and because the Prophet (blessings and peace of Allah be upon him) forbade reviling them; moreover, Allah has forgiven them and told us that He is pleased with them. End quote. "Tafseer al-Qurtubi, 16/321 "
Ibn Abi Zayd al-Qayrawaani said, when discussing what the Muslim is obliged to believe about the Companions of the Messenger of Allah (blessings and peace of Allah be upon him) and what should be said about them: No one of the Companions of the Messenger (blessings and peace of Allah be upon him) should be mentioned except in the best of terms, and we should refrain from discussing the disputes that arose among them. They are the most deserving of people to have their words and actions interpreted in the best manner and to be thought of in the best terms. End quote. 
‘Aqeedat Ahl as-Sunnah wa’l-Jamaa‘ah fi’s-Sahaabah al-Kiraam, 2/734 
Abu ‘Abdullah ibn Battah (may Allah have mercy on him) said, discussing the beliefs of Ahl as-Sunnah wa’l-Jamaa‘ah: After that we should refrain from discussing the disputes that arose among the Companions of the Messenger of Allah (blessings and peace of Allah be upon him), for they witnessed great events with him and were the first people to attain virtue; Allah has forgiven them and has instructed us to pray for forgiveness for them and to draw close to Him by means of loving them, as He has enjoined on the lips of His Prophet. He knew what would happen among them and that they would fight; however they were given precedence over the rest of mankind because mistakes, whether deliberate or otherwise, were already forgiven for them, and they have been forgiven for all disputes that arose among them. 
Kitaab ash-Sharh wa’l-Ibaanah ‘ala Usool as-Sunnah wa’d-Diyaanah, p. 268 
Abu ‘Uthmaan as-Saaboni said, discussing the belief of the Salaf and scholars of hadeeth: 
They believed that we should refrain from discussing the disputes that arose among the Companions of the Messenger of Allah (blessings and peace of Allah be upon him) and that we should avoid saying anything that may be construed as criticism of them; we should pray for mercy for all of them and love all of them. 
‘Aqeedat as-Salaf wa Ashaab al-Hadeeth, in Majmoo‘at ar-Rasaa’il al-Muneeriyyah, 1/129 
Shaykh al-Islam Ibn Taymiyah said: 
One of the basic principles of Ahl as-Sunnah wa’l-Jamaa‘ah is that they think of and speak of the Companions of the Messenger of Allah (blessings and peace of Allah be upon him) in the best terms, as Allah, may He be exalted, has described them (interpretation of the meaning):
“And those who came after them say: Our Lord! Forgive us and our brethren who have preceded us in Faith, and put not in our hearts any hatred against those who have believed. Our Lord! You are indeed full of kindness, Most Merciful” [al-Hashr 59:10]. 
And that is in obedience to the Prophet (blessings and peace of Allah be upon him), who said: “Do not revile my Companions, do not revile my Companions, for by the One Whose hand is my soul, if one of you were to spend the equivalent of Uhud in gold [in charity], it would not be equivalent to the mudd [a unit of measure] of one of them or half of that.” 
They accept what is said in the Qur’aan and Sunnah, and are unanimously agreed on their virtues and status. 
They do not believe that any of the Sahaabah are infallible and were protected from major or minor sins; rather it is possible that they may have committed sins in general, but they did a great deal of righteous deeds and attained virtues by means of which they were forgiven for whatever sins they committed, to such an extent that they were forgiven for bad deeds for which no one who came after them was forgiven, because they did good deeds that erased bad deeds, in a manner that was not granted to anyone who came after them. 
Moreover, if one of them did commit a sin, then he would have repented from it, or done good deeds that erased it, or have been forgiven for it by virtue of his being one of the first to come to Islam, or by means of the intercession of Muhammad (blessings and peace of Allah be upon him) of which they are the most deserving of people, or by being tested with a calamity in this world by means of which it was expiated – if this is the case with regard to real sins, then how about the matters in which they based their actions on what they thought best (ijtihaad): if they got it right they will have two rewards and if they got it wrong they will have one reward and the mistake will be forgiven? 
Moreover the amount of objectionable actions that some of them may have done is very little and insignificant in comparison to their virtues and good characteristics such as belief in Allah and His Messenger, jihad for the sake of Allah, migration (Hijrah), supporting (the religion of Allah), beneficial knowledge and righteous deeds. 
Anyone who studies their biographies with knowledge and insight, and sees what Allah bestowed upon them of virtues, will realise for certain that they are the best of people after the Prophets; there never was and there will never be anyone like them. They are the elite of this ummah, which is the best of nations and the dearest of them to Allah, may He be exalted. End quote. "Majmoo‘ al-Fataawa, 3/152-153"
Al-Haafiz Ibn Hajar said: 
Ahl as-Sunnah are unanimously agreed that it is obligatory to object to any slandering any of the Sahaabah because of what happened of that (disputes), even if ones knows which of them was in the right, because they did not fight in those wars except on the basis of what they thought was right (ijtihaad), and Allah, may He be exalted, has forgiven the one who is mistaken in his ijtihaad. Indeed it is proven that he will be given one reward, and the one who gets it right will be given two rewards. End quote "Fath al-Baari, 13/34 "
A great deal of similar comments have been made by the scholars and it is essential to follow that, because this will protect us from making mistakes and it is acknowledging the rights of the best of people after the Prophets. 
As for the killing of ‘Uthmaan and ‘Ali (may Allah be pleased with them both), there is no doubt that they were killed unlawfully and we testify that they will be in Paradise, and we testify that the one who did this action, or played any part in it, or approved of it, or knew of it and did not denounce it, were led astray in the worst of ways and led others astray, and was a man of evil, who followed a path other than that of the believers; Allah will bring him to account on the Day of Resurrection for his actions and Allah, may He be exalted, will judge between them on the Day of Resurrection. "Say (O Muhammad (peace and blessings of Allah be upon him)): O Allah! Creator of the heavens and the earth! All-Knower of the Ghaib (unseen) and the seen. You will judge between Your slaves about that wherein they used to differ" [az-Zumar 39:46].