Musnad Ahmad 32:1755

'Affan narrated to us, Abu 'Awanah narrated to us, 'Abd al-Rahman ibn al-Asamm narrated to us, he said: Anas was asked about the *takbir* (saying "Allahu Akbar") in prayer while I was listening, and he said: "He says *takbir* when he bows, and when he prostrates, and when he raises his head from prostration, and when he stands between the two rak'ahs." He said: Hakim said to him, "From whom do you memorize this?" He said: "From the Messenger of Allah, peace and blessings of Allah be upon him, and Abu Bakr, and 'Umar." Then he remained silent. Hakim said to him: "And 'Uthman?" He said: "And 'Uthman."