It is reported about Sayyidah Aishah al-Siddiqah (may Allah be pleased with her) that, while mentioning the Khawarij, she asked: "Who killed them?" The people told her: "Sayyiduna Ali (may Allah be pleased with him)." Sayyidah Aishah (may Allah be pleased with her) said: I heard the Prophet (peace and blessings be upon him) say: "The best people of my Ummah will kill them, and they (that is, the Khawarij) are the worst people of my Ummah."
Al-Bazzar recorded it. Its chain contains the narrator Ata' bin al-Sa'ib, whose memory became confused (ikhtilat).
Al-Tabarani recorded a similar narration in al-Mu'jam al-Awsat, in which the whole incident is related.
Hadith Referenceمجمع الزوائد ومنبع الفوائد / كتاب قتال أهل البغي / 10447