سنن النسائي

It was narrated from Anas bin Malik that the Messenger of Allah (ﷺ) rode a horse and fell from it, and sustained an injury on his right side. He led one of the prayers sitting, and we prayed behind him sitting. When he had finished he said: "The Imam is appointed to be followed. If he prays standing then pray standing; when he bows, bow; when he says, Sami' Allahu liman hamidah (Allah hears those who praise Him), say 'Rabbana lakalhamd (Our Lord, to You be praise); and if he prays sitting then pray sitting, all of you."