Sahih Muslim 1848a

It has been narrated on the authority of Abu Huraira that the Messenger of Allah (ﷺ) said:One who defected from obedience (to the Amir) and separated from the main body of the Muslims - if he died in that state-would die the death of one belonging to the days of Jahiliyya (i.e. would not die as a Muslim). One who fights under the banner of a people who are blind (to the cause for which they are fighting, i.e. do not know whether their cause is just or otherwise), who gets flared up with family pride, calls (people) to fight for their family honour, and supports his kith and kin (i.e. fights not for the cause of Allah but for the sake of this family or tribe) - if he is killed (in this fight), he dies as one belonging to the days of Jahiliyya. Whoso attacks my Ummah (indiscriminately) killing the righteous and the wicked of them, sparing not (even) those staunch in faith and fulfilling not his promise made with those who have been given a pledge of security - he has nothing to do with me and I have nothing to do with him