Musannaf Ibn Abi Shayba 36:514

Waki’ narrated to us, from Nafi’ ibn ‘Umar, from Ibn Abi Mulayka, who said: A man said to Abu Bakr: "O Caliph of Allah." He said: "I am not the Caliph of Allah, but I am the Caliph of the Messenger of Allah, and I am satisfied with that."