Sahih Muslim Book 020, Hadith Number 4555.
Chapter : Instruction to stick to the main body of the Muslims in the time of trials and warning against those inviting people to disbelief.
It has been narrated on the authority of Abu Huraira that the Messenger of Allah (may peace be upon him) 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 ate 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. fight 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 Jhiliyya. Whoso attacks my Umma (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.