Sahih Muslim 1066 a
The Book of Zakat
Exhortation to kill the Khawarij
'Ali said: Whenever I narrate to you anything from the Messenger of Allah (ﷺ) believe it to be absolutely true as falling from the sky is dearer to me than that of attributing anything to him (the Prophet) which he never said. When I talk to you of anything which is between me and you (there might creep some error in it) for battle is an outwitting. I heard the Messenger of Allah (ﷺ) as saying: There would arise at the end of the age a people who would be young in age and immature in thought, but they would talk (in such a manner) as if their words are the best among the creatures. They would recite the Qur'an, but it would not go beyond their throats, and they would pass through the religion as an arrow goes through the prey. So when you meet them, kill them, for in their killing you would get a reward with Allah on the Day of Judgement.