Musannaf Ibn Abi Shayba 3:4635

Abu Bakr narrated to us, he said: Yahya bin Sa’id narrated to us, from al-‘Awwam bin Hamzah, he said: I asked Abu ‘Uthman about the Qunut. He said: "After the bowing (ruku’)." I said: "From whom?" He said: "From Abu Bakr and ‘Uthman."