Musnad Abu Ya'la al-Mawsili 118:60

Ishaq narrated to us, Sufyan narrated to us, al-Zuhri narrated to us, he heard Anas bin Malik saying: The Messenger of Allah (peace and blessings of Allah be upon him) fell from a horse, and his right side was grazed. We entered to visit him, and the prayer time 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 'Allahu Akbar' (God is the Greatest), then say 'Allahu Akbar'; when he bows, then bow; when he prostrates, then prostrate; and when he says 'Sami' Allahu liman hamidah' (Allah hears those who praise Him), then say 'Rabbana wa lakal-hamd' (Our Lord, and to You is the praise). And when he prays sitting, then pray sitting, all of you." Abu Ya'qub said: Sufyan narrated it to us another time, and he said: "Rabbana wa lakal-hamd."