[Intro: The Khawarij are probably the first group of muslims who deviated from the straight path, For more read “Eeman and its components, and explaining the misguidance of the Khawarij and the Murjiyah‘ from http://www.qsep.com/books/32.php]
Ibn Abbas (radiya Allahu ‘anhu) Narrates:
“When the six thousand-strong rebels gathered to overthrow Ali bin Abu Talib and his supporters from amongst the companions of Rasool’Allah (salla Allahu ‘alayhi wa sallam) , people came to Ali bin Abu Talib to warn him of the imminent danger saying, ‘O Commander of the Believers, those people are certainly going to rebel against you.’ He would answer, ‘Let them do so, for I shall not be the first to raise the sword, and I know that they will raise theirs.’
So I told Ali, ‘O Commander of the Believers, would you delay the congregational salah a little to give me time to go to those people and talk to them?’ He said, ‘I fear for your safety.’ But I assured him, ‘Not if Allah wills,’ and ‘I was a calm person who never harmed anyone.’
I then put on my best outfit (Abu Zameel narrated that Ibn Abbas was a handsome man and an eloquent speaker with a strong voice) and entered the rebels’ dwelling while they were resting during the middayheat. When I came upon them, I found them to be devout worshippers whose hands had become calloused and their faces marked from prolonged contact with the earth during prostration. Their outfits were shaggy and their worn out faces bore the mark of constant night salah vigil.
When I entered they greeted me, ‘Welcome Ibn Abbas. What brought you here and what is that you are wearing?’
I said, ‘What do you see wrong with my outfit? I saw Rasool’Allah, salla Allahu ‘alayhi wa sallam, wearing better outfits and Allah says:
“Say, who has forbidden the adornment from Allah which He produced for His servants and the good (lawful) things of provision? “[7:32]
They then said, ‘What brought you here?’
I replied, ‘I have come to talk to you about the companions of Rasool’Allah, salla Allahu ‘alayhi wa sallam, who are with the son-in-law of Rasool’Allah, salla Allahu ‘alayhi wa sallam. It was amongst them that the Revelation descended and it is they who best know its meaning and behold, there is none of them among you. I was warned against debating with you, for Allah says:
But (in fact) they are a people prone to dispute [43:58]
‘ but a few advised me to come and talk to you.’
I continued, ‘Tell me what you hold against the cousin of Rasool’Allah, salla Allahu ‘alayhi wa sallam, who is also his son-in-law and was one of the first to believe in him, and against the companions who are with him.’
They said, ‘Three grievances.’
I asked them to elaborate and they answered, ‘He authorized human arbitration when Allah says:
… Judgment is not but for Allah… [12:40, etc]
‘what role do humans have when Allah restricted Judgment to Himself?’
I asked them about the second grievance, so they said, ‘He fought in battle but took nor enslaved any captives and captured no spoils. If the ones he fought against were disbelievers then their possessions were his lawful spoils, and if they were believers then why did he shed their blood?’
I then asked them about their third grievance, to which they answered, ‘He forfeited the title of ‘the Commander of the Believers.’ If he is not the commander of the faithful than he is the commander of the unfaithful.’
I enquired from them if they had any other grievances, but they said those were enough. So I asked them if I were to recite ayaat from the book of Allah and narrate to them from the undisputed ahadeeth of Rasool’Allah, salla Allahu ‘alayhi wa sallam, which would refute their arguments, would they rejoin the jamaa’ah and cease their rebellion? They answered in the affirmative.
I said, ‘With respect to your first argument, Allah says:
O you who have believed, do not kill game while you are in the state of ihram…as judged by two just men among you… [5:95]
And He said about a woman and her husband:
Allah the Mighty and Majestic has said concerning differences between the woman and her husband, “If you fear a breach between the twain (the man and his wife), appoint (two) arbitrators, one from his family and the other from hers; if they both wish for peace, Allah will cause their reconciliation. Indeed Allah is Ever All-Knower, Well-Acquainted with all things.” (Qur’an, 4:35)
I ask you, by Allah, what is more important: To allow human arbitration in game and matters of marriage or to allow it in matters of reconciliation and saving the lives of Muslims? And you know very well that Allah could have passed His Judgment on those matters and curbed all human arbitration in the beginning.’
They answered, ‘In matters of reconciliation and saving lives of Muslims.’
I asked, ‘Do you accept my argument?’
They said, ‘By Allah we do.’
I said, ‘Concerning your objection to the fact that he enslaved no captive and captured no spoils, would you have captured your mother, Aisha, radiya Allahu anha? Or would you have made it lawful for yourselves to enslave her? Had you allowed yourself this, you would not be Muslims, and if you claim that she is not the mother of the faithful, then you would not be Muslims either.
Allah said: “The Prophet is more worthy of the believers than themselves, and his wives are their mothers. [33:6]
You reciprocate between two contradicting falsehoods, so select for yourselves. Do you accept this argument?’
They looked at each other and said, ‘By Allah we do.’
I said, ‘As for him forfeiting the title of ‘Commander of the Believers’, I will answer to this grievance with an established precedence that you will find satisfactory. When Rasool’Allah, salla Allahu ‘alayhi wa sallam, asked Quraish to sign the treaty of Hudaibiyah, he, salla Allahu ‘alayhi wa sallam, and both Suhail ibn Amr and Abu Sufyan who were representing the disbelievers, were the chosen representatives of their respective people to sign the treaty. Upon drafting the treaty, Rasool’Allah salla Allahu ‘alayhi wa sallam, instructed Ali to write, ‘This is what Muhammad the Prophet of Allah agreed upon with’ but they (the disbelievers) objected to this saying, ‘had we believed that you were the Prophet of Allah we would not have blocked your entrance to the house of Allah, and we would not have fought with you. Alas, write Muhammad ibn Abdullah.’ He answered, ‘Verily I am the Prophet of Allah, despite your rejection. Write O Ali, Muhammad ibn Abdullah.’ Rasool’Allah, salla Allahu ‘alayhi wa sallam, was immensely more superior than Ali, and yet his forfeiting the title of the Prophet (while signing the treaty) did not invalidate his Prophethood. Do you accept this argument?’
They replied, ‘By Allah, we do.’”
Source: [This incident has been related in Musnad Ahmad (1/342), An-Nasa’i in Khasa’is Ali (no. 190), Al-Bayhaqi in Sunan Al-Kubra (8/179)]
Now you may wonder what happened next??
Well, nearly 2000 of them joined Ali radi Allahu anhu after listening to Ibn Abbas and rest of them continue to disagree and revolted. Later Ali fought a battle with them.. Battle of Nahrawan.