Musnad Abu Ya'la al-Mawsili 6:97

Surayj ibn Yunus narrated to us, Abu Mu'awiyah narrated to us, Isma'il narrated to us, from Qays ibn Abi Hazim, from Sa'd, that he stood up during the two rak'ahs (of prayer), so they glorified Allah (said "SubhanAllah") to him. He said: So he completed the standing. He said: And he performed the two prostrations of forgetfulness when he finished. Then he said: "Do you see me sitting? I only did as I saw the Messenger of Allah, peace and blessings of Allah be upon him, do."