Musnad Ahmad 830:10

Waki' narrated to us, Uthman Abu Salamah al-Shahham narrated to us, Muslim ibn Abi Bakrah narrated to me, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "There will emerge a people, young in age, foolish in intellect, harsh, their tongues fluent with the Quran, they recite it, but it does not pass beyond their collarbones. So if you meet them, put them to sleep (neutralize them), and then if you meet them, kill them, for the one who kills them will be rewarded."