صحيح مسلم

It has been narrated (through a different chain of transmitters) on the authority of Abu Huraira that the Messenger of Allah (ﷺ) said: Whoever defects from obedience (to the Amir) and separates from the main body of the Muslim - and dies in that state - dies the death of one belonging to the days of jahiliyya. And he who is killed under the banner of a man who is blind (to the cause for which he is fighting), who gets flared up with family pride and fights for his tribe is not from my Ummah, and whosoever from my followers attacks my followers (indiscriminately) killing the righteous and the wicked of them, sparing not (even) those staunch in faith and fulfilling not his obligation towards them who have been given a pledge (of security), is not from me (i.e. is not my follower).