Musnad Abu Ya'la al-Mawsili 150:303

Musa narrated to us, Abd al-Wahhab narrated to us, Ayyub narrated to us, from Hisham ibn Urwah, from his father, from Aisha: That the Messenger of Allah (may Allah bless him and grant him peace) prayed with the people during his illness while he was sitting. They stood, so he gestured to them, and they sat. Then he said: "The Imam is only to be followed. So when he bows, then bow; and when he prostrates, then prostrate; and when he prays sitting, then pray sitting."