Musannaf Ibn Abi Shayba 38:158

Ibn Numayr narrated to us, he said: Abd al-Aziz ibn Siyah narrated to us, he said: Habib ibn Abi Thabit narrated to us, from Abu Wa’il, who said: I came to him and asked him about those people whom Ali killed. He said: I said, "Why did they separate from him? And what did they respond to him with? And what did he call them to? And why did they separate from him, then he deemed their blood lawful?" He said: "When the killing intensified among the people of al-Sham at Siffin, Muawiyah and his companions took refuge with a rope (or mountain). Amr ibn al-Aas said, 'Send the Mushaf to Ali, for by Allah, he will not reject it from you.' He said: So a man came carrying it, calling out, 'Between us and you is the Book of Allah: *Have you not seen those who were given a portion of the Scripture, being invited to the Book of Allah to judge between them, then a party of them turns away while they are refusing?*' (3:23). He said: Ali said, 'Yes, between us and you is the Book of Allah; I am more entitled to it than you.' He said: Then the Khawarij came—and we used to call them the 'Reciters' (al-Qurra) in those days—and they came with their swords on their shoulders. They said, 'O Commander of the Faithful, we will not walk toward 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 be upon him) on the day of al-Hudaybiyyah, and if we had seen fighting, we would have fought. That was during the peace treaty that was between the Messenger of Allah (peace be upon him) and the polytheists. Umar came and went to the Messenger of Allah (peace be upon him) and said, 'O Messenger of Allah, are we not on the truth and they on falsehood?' He said, 'Yes.' He said, 'Are our dead not in Paradise and their dead 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 be lost.' He said: Umar left, not patient, filled with 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 dead not in Paradise and their dead 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 be lost.' He said: Then the Quran was revealed to Muhammad (peace be upon him) regarding the 'Victory' (al-Fath). He sent for Umar and read it to him. He said, 'O Messenger of Allah, is it a victory?' He said, 'Yes.' So his soul was at peace and he returned.' Ali said, 'O people, this is a victory,' so he accepted the arbitration and returned, and the people returned. Then they went out to Harura’—that group of the Khawarij, more than ten thousand. He sent to them, adjuring them by Allah, but they refused him. Sa’sa’ah ibn Suhan came to them and adjured them by Allah and said, 'Why do you fight your Caliph?' They said, 'We fear tribulation.' He said, 'Do not hasten the misguidance of a year for fear of the tribulation of the coming year.' They returned and said, 'We will go our own way; if Ali accepts the arbitration, we will fight them on the day of Siffin, and if he breaks it, we will fight alongside him.' They marched until they reached al-Nahrawan. A faction separated from them and began threatening people with killing. 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, 'Do you see? Will you march to the people of al-Sham, or will you return to these who have remained behind to your children?' They said, 'No, rather we will return to them.' He mentioned their matter and narrated what the Messenger of Allah (peace be upon him) had said about them: 'A group 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 and said, 'O people, if you are only fighting 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 let this not be your fighting.' The people launched a single charge, and the cavalry cleared away from them, and they were lying face down. Ali said, 'Seek the man among them.' The people searched 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 teared up. He called for his mount, rode it, and set off until he came to a depression where there were dead bodies, some on top of others. He began pulling them by their feet until he found the man beneath them. They informed him, and Ali said, 'Allahu Akbar.' The people rejoiced and returned. Ali said, 'I will not go on a military expedition this year.' He returned to Kufa, was killed, and Hasan was appointed as his successor, and they followed the path of his father in pledging allegiance to Muawiyah."