Ibn Abbaas (radiyallaahu-anhumaa) said to the khawaarij:

”I have come to you from the companions of the Prophet (sallal-laahu-alayhi-wasallam)-from the Muhajiroun and the Ansar, and from the son of the uncle of the Prophet (sallal-laahu-alayhi-wasallam). The Qur’an was sent down upon them (i.e. in their time) and they are more knowledgeable concerning it than yourselves, and there is not a single one of them amongst you; and so that I may inform you of what they say and inform them of what you say. So some of them (i.e. khawaarij) said: Do not debate with the Quraish, for indeed Allaah said: [بَلْ هُمْ قَوْمٌ خَصِمُونَ] Nay! But they are a quarrelsome people [Soorah Az-Zukhruf Ayah 58]

Ibn Abbas said: I have never seen a people who exerted themselves in worship more than them (i.e. the khawaarij). Their foreheads were scared due to prostration (in prayer at night)….etc Then some of them said: We shall speak to him and see what he says. [After the debate two-thousand of them left their misguidance and the rest remained upon their misguidance. They came out fighting, so the Muhaajiroon and the Ansaar fought them]. [1]

Shaikh Saaleh al-Fawzaan (may Allaah preserve him) said:

The khawaarij were not misguided and misguiding others with their intellects and fear of Allaah, except that they disassociated themselves from the scholars and depended on their own understanding and that of those similar to them. They took instructions from one another, and thus they caused harm to themselves and to the ummah. This is the result of disassociating from the scholars. [2]

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[1] Source: See link for full details http://www.sahab.net/forums/index.php?showtopic=143536

[2] Source: Booklet: Tawjeehaat Muhimmah Li-Shabaabil Ummah by Shaikh Saaleh Al-Fawzaan]