Sahih Muslim Book 20, Hadith Number 4567.

Chapter : Decision about one who tries to disrupt the unity of the Muslims.

It has been narrated (through a still different chain of transmitters) on the same authority (i.e. ‘Arfaja) who said similarly-but adding: “Kill all of them.” I heard the Messenger of Allah (may peace be upon him) say: When you are holding to one single man as your leader, you should kill who seeks to undermine your solidarity or disrupt your unity.