Musnad Ahmad 32:134

Sufyan narrated to us, from al-Zuhri, who heard it from Anas, who said: The Prophet, peace and blessings of Allah be upon him, fell from a horse, and his right side was grazed. We entered upon him to visit him, and the prayer time arrived, so he prayed sitting, and we prayed sitting. When he finished the prayer, he said: "The Imam is only to be followed. So when he says 'Allahu Akbar,' say 'Allahu Akbar,' and when he bows, bow." And Sufyan said once: "And when he prostrates, prostrate, and when he says 'Sami' Allahu liman hamidah,' say 'Rabbana wa lakal-hamd.' And if he prays sitting, then pray sitting, all of you."