Musnad Ahmad 830:73

Rawh narrated to us, Uthman al-Shahham narrated to us, Muslim ibn Abi Bakrah narrated to us, and he was asked: "Did you hear anything regarding the Khawarij?" He said: I heard my father, Abu Bakrah, saying from the Prophet of Allah, peace and blessings of Allah be upon him: "Behold, there will emerge from my nation people who are harsh and sharp-tongued, their tongues fluent with the Quran, yet it does not pass beyond their collarbones. Behold, if you see them, then put them to sleep [kill them], and then if you see them, put them to sleep, for the one who kills them is rewarded."