Sahih Bukhari, Chapter: Shortening the Prayers (At-Taqseer)

Allah's Apostle prayed in his house while sitting during his illness and the people prayed behind him standing and he pointed to them to sit down. When he had finished the prayer, he said, "The Imam is to be followed and so when he bows you should bow; and when he lifts his head you should also do the same."