Sahih Bukhari Volume 9, Book 88, Hadith 6932

حَدَّثَنَا عُمَرُ بْنُ حَفْصِ بْنِ غِيَاثٍ، حَدَّثَنَا أَبِي، حَدَّثَنَا الأَعْمَشُ، حَدَّثَنَا خَيْثَمَةُ، حَدَّثَنَا سُوَيْدُ بْنُ غَفَلَةَ، قَالَ عَلِيٌّ ـ رضى الله عنه ـ إِذَا حَدَّثْتُكُمْ عَنْ رَسُولِ اللَّهِ صلى الله عليه وسلم حَدِيثًا فَوَاللَّهِ، لأَنْ أَخِرَّ مِنَ السَّمَاءِ أَحَبُّ إِلَىَّ مِنْ أَنْ أَكْذِبَ عَلَيْهِ، وَإِذَا حَدَّثْتُكُمْ فِيمَا بَيْنِي وَبَيْنَكُمْ فَإِنَّ الْحَرْبَ خَدْعَةٌ، وَإِنِّي سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ ‏"‏ سَيَخْرُجُ قَوْمٌ فِي آخِرِ الزَّمَانِ، حُدَّاثُ الأَسْنَانِ، سُفَهَاءُ الأَحْلاَمِ، يَقُولُونَ مِنْ خَيْرِ قَوْلِ الْبَرِيَّةِ، لاَ يُجَاوِزُ إِيمَانُهُمْ حَنَاجِرَهُمْ، يَمْرُقُونَ مِنَ الدِّينِ كَمَا يَمْرُقُ السَّهْمُ مِنَ الرَّمِيَّةِ، فَأَيْنَمَا لَقِيتُمُوهُمْ فَاقْتُلُوهُمْ، فَإِنَّ فِي قَتْلِهِمْ أَجْرًا لِمَنْ قَتَلَهُمْ يَوْمَ الْقِيَامَةِ ‏"‏‏

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."