Abdullah Ibn Abbas narrates his debate when he approached the Khawaarij:

“I have come to you from the companions of the Messenger of Allah (peace be upon him), from the Muhajireen and the Ansar, so that I may inform you of what they say.

The Qur’an was sent down upon them. And they are more knowledgeable concerning the revelation than you, and it was revealed amongst them. And none of them are amongst you.”

So some of them said, “Let us speak with him and let us see what he says.” I said, “Inform me of that which you harbor against the son of the uncle of the Messenger of Allah (peace be upon him) and his son-in-law (Ali), the Muhajireen and the Ansar.”

They said, “Three issues.” I asked, “What are they?” They said, “As for the first of them, then he (Ali) has allowed men to judge in the affair of Allah, yet Allah said,

“…The judgement is for none but Allah…” (Qur’an, 12:40)

So it is not for men and it is not for judges.”

So I asked, “This is only one issue.”

They said, ‘As for the other issues, then he fought his enemies, yet he did not take any prisoners or war booty. So if those whom he fought were disbelievers, then their imprisonment and war booty has been made lawful. And if they were believers, then fighting them is not permissible.”

I said, “These are only two issues, so what is the third?”

They said, “Indeed, he erased the title, ‘Leader of the Believers’ from himself, so therefore he is the leader of the disbelievers.” I said, “Do you have anything besides this?” They said, “This is sufficient for us.”

So I said to them, “Do you hold that if I recite to you from the Book of Allah and the Sunnah of His Messenger (peace be upon him), that which refutes your statement, that you will obey?”

They said, “Yes!” So I said, “As for your statement that he has allowed men to judge concerning from the affair of Allah, then I shall recite to you that which shows that Allah relegated His judgment to men.

Allah the Mighty and Majestic has said concerning the woman and her husband,

” If you fear a breach between them twain (the man and his wife), appoint (two) arbitrators, one from his family and the other from her’s; if they both wish for peace, Allâh will cause their reconciliation. Indeed Allâh is Ever All-­Knower, Well­-Acquainted with all things.” (Qur’an, 4:35)

So Allah made the judgement of the men a tradition to be believed in. So do you leave this (issue)?” They said, “Yes!”

Ibn Abbas continued, “As for your statement that he fought and he did not take prisoners, nor did he take war booty, then do you revile your mother, Ayesha? Then you must declare her lawful in that which you have declared lawful with regards to other than her! So if you were to do that, then you would become disbelievers, as she is your mother. And if you were to say that she is not our mother, then you have disbelieved. Since, Allah says,

“The Prophet is closer to the Believers than their ownselves, and his wives are their mothers…” (Qur’an, 33:6]

So you are revolving around two positions of misguidance. Regardless of which of them you adopt, you have adopted misguidance.”

So they began to look at each other. I said, “Have you left from this?” They said, “Yes!”

“And as for your statement, that he erased the title of ‘Leader of the Believers’ from his name, then I shall come to you with one whom you are pleased with and I shall see you. Indeed, you have heard that on the day of Al-Hudaybiyyah, the Prophet (peace be upon him) made a treaty with Suhayl Ibn ‘Amr and Abaa Sufyaan Ibn Harb.

So the Messenger of Allah (peace be upon him) said to the Leader of the Believers (Ali), “Write O Ali: These are the terms of peace agreed upon by Muhammad the Messenger of Allah.” So the pagans said, “No, by Allah! Had we known that you were the Messenger of Allah, we would not have fought you.”

So the Messenger of Allah (peace be upon him) said, “O Allah! You know that I am the Messenger of Allah.

Write O Ali: These are the terms of peace agreed upon by Muhammad Ibn Abdullah.”(Bukhari and Muslim)

So by Allah, the Messenger of Allah was better than Ali, yet he was not removed from Prophethood when he erased the title from his name.”

Abdullah Ibn Abbas then said, “So two thousand of those people repented, and the rest were killed upon misguidance.”

– This incident has been related in Musnad Ahmad (1/342), An-Nisaa’ee in Khasaa’is Ali (no. 190), Al-Bayhaqee in as-Sunanul-Kubraa (8/179) and many others.