Musnad Ahmad 788:236

Hassan narrated to us, Zuhair narrated to us, from Abu Ishaq, from Burayd ibn Abi Maryam, from a man from Banu Tamim, from Abu Musa al-Ash'ari, who said: "Ali ibn Abi Talib, may Allah be pleased with him, led us in a prayer that reminded us of a prayer we used to pray with the Messenger of Allah, may Allah bless him and grant him peace. Either we had forgotten it, or we had abandoned it intentionally; he would say 'Allahu Akbar' upon every rising, lowering, standing, and sitting."