Sahih Muslim Book 020, Hadith Number 4483.
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 Amir b. Sa'd b. Abu Waqqas who said: I wrote (a letter) to Jabir b. Samura and sent it to him through my servant Nafi', asking him to inform me of something he had heard from the Messenger of Allah (may peace be upon him). He wrote to me (in reply): I heard the Messenger of Allah (may peace be upon him) say on Friday evening, the day on which al-Aslami was stoned to death (for committing adultery): The Islamic religion will continue until the Hour has been established, or you have been ruled over by twelve Caliphs, all of them being from the Quraish; also heard him say: A small force of the Muslims will capture the white palace, the police of the Persian Emperor or his descendants. I also heard him say: Before the Day of Judgment there will appear (a number of) impostors. You are to guard against them. I also heard him say: When God grants wealth to any one of you, he should first spend it on himself and his family (and then give it in charity to the poor). I heard him (also) say: I will be your forerunner at the Cistern (expecting your arrival).