Musnad Abu Ya'la al-Mawsili 3:64

Ubayd Allah narrated to us, Waki' narrated to us, al-A'mash narrated to us, from Khaythama, from Suwayd ibn Ghafala, who said: Ali said: I heard the Messenger of Allah (peace and blessings of Allah be upon him) saying: "There will emerge at the end of time a people young in age and foolish in intellect. They will speak with the best of the speech of the creation, and they will recite the Quran, but it will not pass beyond their throats. So if you encounter them, then kill them, for indeed in killing them there is a reward for whoever kills them with Allah on the Day of Resurrection."