Musnad Ahmad 1:63

Muhammad ibn Yazid narrated to us, Nafi‘ ibn ‘Umar al-Jumahi narrated to us, from ‘Abdullah ibn Abi Mulaykah, who said: It was said to Abu Bakr: "O Caliph of Allah." He said: "Rather, I am the Caliph of Muhammad, may Allah bless him and grant him peace, and I am content with that."