Sahih Bukhari Volume 7, Book 70, Hadith Number 562.
Narrated By ‘Aisha : During the ailment of the Prophet some people came to visits him. He led them in prayer while sitting. but they prayed standing, so he waved to them to sit down. When he had finished the prayer, he said, “An Imam is to be followed, so when he bows, you should bow. and when he raises his head, you should raise yours, and if he prays sitting. you should pray sitting.” Abu Abdullah said Al-Humaidi said, (The order of) “This narration has been abrogated by the last action of the Prophet as he led the prayer sitting, while the people prayed standing behind him.”