Jami` at-Tirmidhi 375

Abdullah bun Shaqiq narrated from Aishah, he said:"I asked her about the voluntary Salat of Allah's Messenger (S), she said: 'He would pray long into the night standing, and long into the night sitting. So when he recited and he was standing, he would bow and prostrate whole he was standing, and when he recited while he was sitting, he bowed and prostrated while he was sitting