Musnad Abu Ya'la al-Mawsili 3:213

Abu Bakr ibn Abi Shaybah narrated to us, Abdullah ibn Numayr narrated to us, Abd al-Aziz ibn Siyaah narrated to us, Habib ibn Abi Thabit narrated to us, from Abu Wa’il, who said: I came to him [Ali] and asked him about those people whom Ali killed. I said: "Why did they separate from him? Why did they deem [the shedding of] their blood lawful? Why did he call them? Why did they separate from him, and by what did he deem their blood lawful?" He said: "When the killing intensified among the people of al-Sham at Siffin, Mu’awiyah and his companions took refuge in a ruse. Amr ibn al-Aas said to him: 'Send the Mushaf (copy of the Quran) to me, for by Allah, we will not return it to you.' A man came carrying it and called out: 'Between us and you is the Book of Allah. *Have you not seen those who were given a portion of the Book...* [Al-Imran: 23].' Ali said: 'Yes, between us and you is the Book of Allah; we are more entitled to it than you.' Then the Khawarij came—and we used to call them 'the reciters' in those days—and they came with their swords upon their shoulders and said: 'O Commander of the Faithful, will you not march against these people until Allah judges between us and them?' Sahl ibn Hunayf stood up and said: 'O people, suspect your own selves. We were with the Messenger of Allah (peace and blessings of Allah be upon him) on the day of al-Hudaybiyah, and if we had seen fighting, we would have fought. That was during the peace treaty that was between the Messenger of Allah (peace and blessings of Allah be upon him) and the polytheists. Umar ibn al-Khattab came and said: 'O Messenger of Allah, are we not on the truth and they on falsehood?' He said: 'Yes.' He said: 'Are our killed not in Paradise and their killed in the Fire?' He said: 'Yes.' He said: 'Then why do we accept humiliation in our religion and return while Allah has not judged between us and them?' He said: 'O son of al-Khattab, I am the Messenger of Allah, and Allah will never let me down.' Umar left, unable to contain his rage, until he came to Abu Bakr and said: 'O Abu Bakr, are we not on the truth and they on falsehood?' He said: 'Yes.' He said: 'Are our killed not in Paradise and their killed in the Fire?' He said: 'Yes.' He said: 'Then why do we accept humiliation in our religion and return while Allah has not judged between us and them?' He said: 'O son of al-Khattab, he is the Messenger of Allah, and Allah will never let him down.' Then the Quran was revealed to Muhammad regarding the victory (al-Fath), so he sent for Umar and recited it to him. He said: 'O Messenger of Allah, is it a victory?' He said: 'Yes.' He said: 'Then his soul was at peace, and he returned, and the people returned.' Then they (the Khawarij) went out to Harura’, that group of Khawarij, numbering more than ten thousand. Ali sent to them, adjuring them by Allah, but they refused him. Sa’sa’ah ibn Suhan came to them and adjured them, saying: 'Why do you fight your Caliph?' They said: 'For fear of discord (fitnah).' He said: 'Do not hasten the misguidance of a year for fear of the discord of a year to come.' They returned and said: 'We will proceed upon what we came for; if Ali accepts the arbitration, we will fight him as we fought on the day of Siffin, and if he annuls it, we will fight alongside him.' They marched until they reached al-Nahrawan. A faction separated from them and began to kill people at night. Their companions said: 'Woe to you! It is not for this that we separated from Ali.' The matter reached Ali, so he stood and addressed the people, saying: 'What do you think? Shall we march to the people of al-Sham, or shall we return to these people who have turned against your families?' They said: 'Rather, we return to them.' He mentioned their matter and narrated what the Messenger of Allah (peace and blessings of Allah be upon him) had said about them: 'A faction will emerge during a time of disagreement among the people; the closer of the two parties to the truth will kill them. Their sign is a man among them whose hand is like the breast of a woman.' They marched until they met at al-Nahrawan and fought a fierce battle. Ali’s cavalry could not stand against them. Ali stood up and said: 'O people, if you are fighting only for my sake, then by Allah, I have nothing with which to reward you. But if you are fighting for the sake of Allah, then this should not be your conduct.' The people launched a single charge, and the cavalry cleared away from them, and they were lying face down. Ali said: 'Search for the man among them.' The people searched for the man but did not find him, until some of them said: 'Ibn Abi Talib has deceived us regarding our brothers until we killed them.' Ali’s eyes welled with tears. He called for his mount, rode it, and set off until he came to a depression where the killed were piled one upon another. He began pulling them by their feet until he found the man beneath them. They informed him, and Ali said: 'Allahu Akbar,' and he rejoiced, and the people rejoiced. They returned, and Ali said: 'I will not wage war this year.' He returned to Kufa, and he was killed—may Allah have mercy on him—and Hasan was appointed as his successor. He followed the path of his father, then he sent the pledge of allegiance to Mu’awiyah."