Sahih Muslim Book 20, Hadith Number 4543.

Chapter : Fulfilment of the covenant made with the Caliphs is imperative, the caliph to whom allegiance is sworn in the first instance has an established supremacy over those who assume powers.

It has been narrated by Abu Huraira that the Holy Prophet (may peace be upon him) said: Banu Isra’il were ruled over by the Prophets. When one Prophet died, another succeeded him; but after me there is no prophet and there will be caliphs and they will be quite large in number. His Companions said: What do you order us to do (in case we come to have more than one Caliph)? He said: The one to whom allegiance is sworn first has a supremacy over the others. Concede to them their due rights (i.e. obey them). God (Himself) will question them about the subjects whom He had entrusted to them.