Sahih Bukhari Volume 001, Book 012, Hadith Number 769.
Narated By Anas bin Malik : Allah's Apostle fell from a horse and the right side of his body was injured. We went to enquire about his health meanwhile it was time for the prayer and he led the prayer sitting and we also prayed while sitting. On completion of the prayer he said, "The Imam is to be followed; say Takbir when he says it; bow when he bows; rise when he rises and when he says "Sami'a-l-lahu Liman hamida," say, "Rabbana walaka-lhamd", and prostrate if he prostrates." Sufyan narrated the same from Ma'mar. Ibn Juraij said that his (the Prophet's) right leg had been injured.