Musnad Ahmad 32:1821

Abu Nu'aym narrated to us, Sufyan narrated to us, from Abd al-Rahman ibn al-Asamm, who said: I heard Anas saying: "The Prophet, peace and blessings of Allah be upon him, and Abu Bakr, and Umar, and Uthman, used to complete the takbir when they raised [their heads from prostration] and when they placed [their foreheads for prostration]."