Musnad Ahmad 32:1692

'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. He said: "He says *takbir* when he bows, when he prostrates, when he raises his head from prostration, and when he stands up 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. He said: Hakim said to him: "And 'Uthman?" He said: "And 'Uthman."