Muwatta Imam Malik (Riwayat Ibn al-Qasim)

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."