Musannaf Ibn Abi Shayba 3:4758

'Abdah narrated to us, from Hisham, from his father, from 'Aisha, who said: The Messenger of Allah (peace and blessings of Allah be upon him) became ill, so some of his companions entered to visit him. The Messenger of Allah (peace and blessings of Allah be upon him) prayed sitting, and they prayed while standing, following his prayer. He signaled to them to sit, so they sat. When he finished, he said: "The Imam was only appointed to be followed. So when he bows, then bow; and when he rises, then rise; and when he prays sitting, then pray sitting."