Musnad Ahmad 32:1382

Hashim narrated to us, Sulayman narrated to us, from Thabit, who said: "Anas ibn Malik described to us the prayer of the Messenger of Allah, may Allah bless him and grant him peace." Then he stood to lead us in prayer. He bowed, then stood upright until some of us thought he had forgotten. Then he prostrated, then sat upright until some of us thought he had forgotten. Then he sat upright.