Musannaf Ibn Abi Shayba 35:86

Ibn ‘Uyaynah narrated to us, from al-Zuhri, who said: I heard Anas ibn Malik saying: The Prophet, may Allah bless him and grant him peace, fell from a horse, and his right side was grazed. We entered upon him to visit him, and the prayer time arrived, so he led us in prayer while sitting, and we prayed behind him while standing. When he finished the prayer, he said: "The Imam was only appointed to be followed. So when he says the takbir, say the takbir; when he bows, bow; when he prostrates, prostrate; when he rises, rise; and when he says 'Allah hears those who praise Him,' say 'O Allah, our Lord, to You is the praise.' And if he prays sitting, then pray sitting, all of you."