Sahih Muslim Book 020, Hadith Number 4481.
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 that the Holy Prophet (may peace be upon him) said: This order will continue to be dominant until there have been twelve Caliphs. The narrator says: Then he said something which I could not understand, and I said to my father: What did he say? My father told me that he said that all of them (Caliphs) would be from the Quraish.