Mukhtasar Bukhari

Narrated Abu Huraira (may Allah be pleased with him) from the Prophet (peace and blessings of Allah be upon him) that he said: "The people of Israel were governed by prophets. Whenever a prophet died, another succeeded him. But there will be no prophet after me; however, there will be caliphs and they will increase in number." The companions asked, "What do you command us to do at that time?" He (peace and blessings of Allah be upon him) said, "Fulfill the pledge of allegiance to the first caliph and then to the one who comes after him. Continue to fulfill their rights (of obedience). Surely, Allah will question them on the Day of Resurrection about how they fulfilled the rights of their subjects."