Ibn Hibban

Sayyiduna Anas (may Allah be pleased with him) narrates: The Prophet (peace and blessings be upon him) mounted a horse and fell from it. His right side was injured. He offered some prayers sitting. We also offered prayer sitting behind him. When he finished the prayer, he said: "The Imam is appointed so that he may be followed. When he offers prayer standing, you should also offer prayer standing. When he goes into bowing (ruku‘), you should also go into bowing. When he rises (from ruku‘), you should also rise. When he recites «سمع اللهُ لِمَنْ حَمِدَهُ», you should recite «رَبَّنَا وَلَكَ الْحَمْدُ». And when he offers prayer sitting, then all of you should also offer prayer sitting."