Musnad Abu Ya'la al-Mawsili 149:140

Abu al-Rabi' al-Zahrani narrated to us, Hammad narrated to us, Hisham narrated to us, from his father, from Aisha, who said: The companions of the Messenger of Allah (may Allah bless him and grant him peace) entered upon him during his illness while he was praying sitting down. They stood to pray behind him, so he gestured to them with his hand to sit, and they sat. When the Prophet (may Allah bless him and grant him peace) finished, he said: "The Imam is only to be followed. So when he bows, then bow; and when he prostrates, then prostrate. If he prays standing, then pray standing; and if he prays sitting, then pray sitting."