Musnad Ahmad 26:281

Yahya ibn Abi Bukayr narrated to us, Abu Bakr ibn Ayyash narrated to us, from Asim, from Zirr, from Abdullah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "A people will emerge at the end of time, foolish of mind, young of age—or he said: young of teeth—they will speak with the best of people’s speech. They will recite the Quran with their tongues, but it will not pass beyond their throats. They will pass through Islam just as an arrow passes through the game. So whoever encounters them, let him kill them, for in killing them there is a great reward with Allah for whoever kills them."