Musnad Ahmad 32:713

'Abd al-Razzaq narrated to us, Ma'mar narrated to us, from al-Zuhri, from Anas, who said: The Prophet, peace and blessings of Allah be upon him, fell from a horse, and his right side was grazed. They entered upon him, and he led them in prayer while sitting, and he signaled to them to sit. When he finished the salutation, he said: "The Imam is only appointed to be followed. So when he says the takbir, say the takbir; when he bows, bow; when he says, 'Allah hears those who praise Him,' say, 'Our Lord, and to You is the praise'; when he prostrates, prostrate; and when he prays sitting, pray sitting, all of you."