Sahih Muslim Book 020, Hadith Number 4478.
Chapter : The people are subservient to the Quraish and the Caliphate is the right of the Quraish.
It has been reported on the authority of Jabir b. Samura who said: I heard the Messenger of Allah (may peace be upon him) say: The affairs of the people will continue to be conducted (well) as long as they are governed by twelve men. Then the Holy Prophet (may peace be upon him) said words which were obscure to me. I asked my father: What did the Messenger of Allah (may peace be upon him) say? He said: All of the (twelve men) will be from the Quraish.