Sahih Muslim Book 1, Hadith Number 179.

Chapter : The utterance of the Apostle (may peace be upon him). He who takes up arms against us is not of us (i.e. he ceases to be a Muslim).

It is narrated on the authority of Abdullah b. Umar who narrates from the Prophet of Allah (may peace be upon him) who said: He who took up arms against us is not of us.