Ibn Hibban

Sayyiduna Anas (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) fell from a horse. His right side was injured. When the time for prayer came, he led us in prayer while sitting. When he completed the prayer, he said: "The Imam is appointed so that he may be followed. When he says the takbir, you also say the takbir. When he goes into ruku, you also go into ruku. When he rises (from ruku), you also rise. And when he recites «سمع اللہ لمن حمدہ», you recite «ربنا ولك الحمد». When he performs the prayer sitting, then all of you should also perform the prayer sitting."