Musnad Ahmad 32:255

Waki’ and Yahya narrated to us, from Sufyan, from ‘Abd al-Rahman ibn al-Asamm, who said: I heard Anas say: "Indeed, Abu Bakr, ‘Umar, and ‘Uthman used to complete the takbir; they would say the takbir when they prostrated, and when they rose." Yahya said: Or when they lowered [themselves], he said: "They said the takbir."