Musnad Ahmad 32:319

Yahya narrated to us, from Sufyan, from Abd al-Rahman ibn al-Asamm, who said: I heard Anas saying: "The Prophet, may Allah bless him and grant him peace, and Abu Bakr, and Umar, and Uthman, used to complete the takbir. They would say the takbir when they prostrated, and when they raised [their heads]." Yahya said: "Or when they lowered [themselves]."