It is narrated from Sayyiduna Ibn Abbas (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever separates from the community of the Muslims by even a hand-span has thrown off the collar of Islam from his neck; whoever dies while there is no imam over him, his death is a death of Jahiliyyah; and whoever dies under a blind (aimless) banner, calling to tribal partisanship or supporting tribal partisanship, his killing is a killing of Jahiliyyah."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 10641