Sahih Muslim Book 020, Hadith Number 4480.
Chapter : The people are subservient to the Quraish and the Caliphate is the right of the Quraish.
It has been narrated on the authority of Jabir b. Samura who said: I heard the Messenger of Allah (may peace be upon him) say: Islam will continue to be triumphant until there have been twelve Caliphs. Then the Holy Prophet (may peace be upon him) said something which I could not understand. I asked my father: What did he say? He said: He has said that all of them (twelve Caliphs) will be from the Quraish.