Musannaf Ibn Abi Shayba 3:4757

Abu Bakr narrated to us, he said: Sufyan ibn 'Uyaynah narrated to us, from al-Zuhri, he said: I heard Anas say: 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 time for prayer arrived, so he led us in prayer while sitting, and we prayed behind him while sitting. 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 says 'Allah hears those who praise Him,' say 'Our Lord, and to You is the praise'; and when he prays sitting, pray all of you sitting."