Musnad Ahmad 30:842

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, from Furat, I heard Abu Hazim say: I sat with Abu Hurairah for five years, and I heard him narrating from the Prophet, may Allah bless him and grant him peace, that he said: "The Children of Israel were governed by the prophets; whenever a prophet died, another succeeded him. And there is no prophet after me. There will be caliphs, and they will be many." They said: "What do you command us?" He said: "Fulfill the pledge of allegiance to the first, then the first. Give them their right which Allah has assigned to them, for Allah will ask them about what He entrusted to their care."