Sahih Muslim Book 5, Hadith Number 2320.

Chapter : Exhortation to kill the Khwarij.

This Hadith has been narrated through another chain of transmitters and (the narrator) made a mention of elevated forehead, but he made no mention of tucked-up loin cloth and made this addition: “There stood up ‘Umar b. Khattab (Allah be pleased with him), and said: Should I not strike his neck? Upon this he said: No. Then he turned away, and Khalid the Sword of Allah stood up against him, and said: Prophet of Allah, shall I not strike off his neck? He said, No, and then said: A people would rise from his progeny who would recite the Book of Allah glibly and fluently. ‘Umar said: I think he (the Holy Prophet) also said this: If I find them I would certainly kill them like Thamud.”