Musnad Ahmad 32:409

'Abd al-Rahman ibn Mahdi narrated to us, from Sufyan, from 'Abd al-Rahman ibn al-Asamm, from Anas ibn Malik: "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] and when they placed [their foreheads on the ground]."