Musnad Ahmad 1:58

Musa ibn Dawud narrated to us, Nafi'—meaning Ibn 'Umar—narrated to us, from Ibn Abi Mulaykah, who said: It was said to Abu Bakr: "O Caliph of Allah." He replied: "I am the Caliph of the Messenger of Allah, may Allah bless him and grant him peace, and I am content with that."