Musnad Ahmad 5:55

Abu Mu'awiyah narrated to us, Al-A'mash narrated to us, from Khaythamah, from Suwayd ibn Ghaflah, who said: Ali said: "If I narrate to you a hadith from the Messenger of Allah (peace and blessings of Allah be upon him), then for me to fall from the sky is more beloved to me than to lie about him. And if I narrate to you from other than him, then I am a man at war, and war is deception. I heard the Messenger of Allah (peace and blessings of Allah be upon him) say: 'There will emerge at the end of time a people young in age, foolish in intellect. They will speak with the best of the speech of creation, but their faith will not pass beyond their throats. So wherever you encounter them, kill them, for in their killing is a reward for whoever kills them on the Day of Resurrection.'"