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."