Musnad Ahmad 1153:652

Muhammad ibn Abi ‘Adi narrated to us, from Humayd, from ‘Abdullah ibn Shaqiq, who said: I asked the Mother of the Believers about the prayer of the Messenger of Allah, peace and blessings of Allah be upon him, at night. She said: "He would pray a long night standing, and a long night sitting. So when he recited while standing, he would bow while standing, and when he recited while sitting, he would bow while sitting."