Sayyiduna Ali (may Allah be pleased with him) mentioned the Khawarij, or the people of Nahrawan, and said: "Among them is a man with a deficient hand, or a fleshy hand. Were it not that you would become arrogant, I would tell you what Allah has promised, on the tongue of Muhammad (peace and blessings be upon him), to those who fight them." I said: Did you hear this from Muhammad (peace and blessings be upon him)? He said: Yes, by the Lord of the Ka'bah! He said it three times.
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16699