Muwatta Imam Malik (Riwayat Ibn al-Qasim)

It is narrated from Sayyiduna Anas bin Malik (may Allah be pleased with him) that the Messenger of Allah (peace and blessings be upon him) once mounted a horse and fell from it, injuring his right side. Then he led one of the prayers sitting, and we also prayed that prayer sitting behind him. When he turned towards us (after finishing), he said: "The imam is appointed so that he may be followed. When he prays standing, you should also pray standing, and when he bows (in ruku‘), you should bow. When he rises (from ruku‘), you should also rise. When he says «سمع الله لمن حمده», you should say «ربنا ولك الحمد», and when he prays sitting, you should all pray sitting."

And with this same chain of narration (it is narrated from Sayyida Aisha, may Allah be pleased with her) that the Messenger of Allah, peace and blessings be upon him, led the prayer while sitting in his house, and he was ill. The people began to stand and pray behind him, so the Messenger of Allah, peace and blessings be upon him, gestured to them to sit down. Then, when he finished the prayer, he said: "The imam is appointed so that he may be followed; when he bows, you should bow; when he raises his head, you should raise your heads; and when he prays sitting, you should also pray sitting."