Someone asked Muslim bin Abi Bakrah (may Allah have mercy on him): Have you heard anything about the Khawarij? He replied: I heard my father, Sayyiduna Abu Bakrah (may Allah be pleased with him), narrating from the Prophet (peace and blessings be upon him): "There will come in my ummah people who are harsh and fierce, their tongues fluent with the Qur'an, but the Qur'an will not go beyond their throats. When you see them, put them to sleep (i.e. kill them); then when you see them, put them to sleep; and the one who kills them will be rewarded."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب قتال أهل البغي / 16780