Sahih Muslim Book 20, Hadith Number 4476.

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 ‘Abdullah that the Messenger of Allah (may peace be upon him) said: The Caliphate will remain among the Quraish even if only two persons are left (on the earth),