Musnad Ahmad 788:10

Yahya ibn Adam narrated to us, Isra'il narrated to us, from Abu Ishaq, from al-Aswad, who said: Abu Musa said: Ali ibn Abi Talib reminded us of a prayer that we used to perform with the Messenger of Allah, may Allah bless him and grant him peace, which we had either forgotten or abandoned intentionally: "He would say the takbir whenever he bowed, whenever he rose, and whenever he prostrated."