Sahih Muslim
The Book on Government (Kitab Al-Imara)
Book Number: 020 / Hadith Number: 4481
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.