Sahih Bukhari | Volume 4 | Hadith Number 661

Narrated 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."