Musannaf Ibn Abi Shayba 37:630

Abu Mu'awiyah narrated to us, from al-A'mash, from Ibrahim, from Hammam, who said: A man from the People of the Book came to Umar and said, "Peace be upon you, O King of the Arabs." Umar said, "And is this how you find it in your Book? Do you not find the Prophet, then the Caliph, then the Commander of the Believers, then the kings after that?" He said to him, "Yes."