Musnad Abu Ya'la al-Mawsili 118:23

Abu Bakr ibn Abi Shaybah and Abu Khaythamah narrated to us, they said: Ibn ‘Uyaynah narrated to us, from al-Zuhri, from Anas, who said: The Prophet (may Allah bless him and grant him peace) fell from a horse, and his right side was grazed. The time for prayer arrived, so he led us in prayer while sitting. When he finished his prayer, he said: "The Imam is only appointed to be followed. So when he says 'Allahu Akbar' (God is the Greatest), say 'Allahu Akbar'. When he bows, bow. When he rises, rise. When he says 'Sami‘a Allahu liman hamidah' (Allah hears those who praise Him), say 'Rabbana wa laka al-hamd' (Our Lord, and to You is the praise). And when he prays sitting, pray sitting, all of you."