Sahih Bukhari Volume 4, Book 56, Hadith Number 661.

Narated By Abu Huraira : The Prophet said, “The Israelis used to be ruled and guided by prophets: Whenever a prophet died, another would take over his place. There will be no prophet after me, but there will be Caliphs who will increase in number.” The people asked, “O Allah’s Apostle! What do you order us (to do)?” He said, “Obey the one who will be given the pledge of allegiance first. Fulfil their (i.e. the Caliphs) rights, for Allah will ask them about (any shortcoming) in ruling those Allah has put under their guardianship.”