Musnad Ahmad 1153:294

Ibn Numayr narrated to us, from Hisham, from his father, from Aisha, that the Messenger of Allah (peace and blessings of Allah be upon him) prayed during his illness while he was sitting. He prayed while behind him were people standing, so he signaled to them to sit. When he finished his prayer, he said: "The Imam is only to be followed. So when he bows, then bow; and when he rises, then rise; and when he prays sitting, then pray sitting."