Musnad Abu Ya'la al-Mawsili 148:31

Zuhayr narrated to us, Abd al-Rahman narrated to us, from Sufyan, from Abd al-Rahman ibn al-Asamm, from Anas: That the Prophet (peace and blessings of Allah be upon him), Abu Bakr, Umar, and Uthman used to complete the takbir when they raised [their heads] and when they placed [their foreheads on the ground].