Musnad Ahmad 1153:387

Aswad narrated to us, Hammad bin Zayd narrated to us, from Hisham, from his father, from Aisha, who said: When the Prophet, peace and blessings of Allah be upon him, became ill, his companions entered upon him to visit him, and they stood. He gestured to them to sit. When he finished his prayer, he said: "The Imam is to be followed. So when he says the Takbir, say the Takbir; when he bows, bow; when he prays sitting, pray sitting; and when he prays standing, pray standing."