Sahih Bukhari | Volume 9 | Hadith Number 64
Narrated by 'Ali
Whenever I tell you a narration from Allah's Apostle, by Allah, I would rather fall down from the sky than ascribe a false statement to him, but if I tell you something between me and you (not a Hadith) then it was indeed a trick (i.e. I may say things just to cheat my enemy). No doubt I heard Allah's Apostle saying, "During the last days there will appear some young foolish people who will say the best words but their faith will not go beyond their throats (i.e. they will have no faith) and will go out from (leave) their religion as an arrow goes out of the game. So, where-ever you find them, kill them, for who-ever kills them shall have reward on the Day of Resurrection."