Musnad Ahmad 1153:1974

Yazid narrated to us, he said: Humayd informed us, from Abdullah ibn Shaqiq, he said: I said to Aisha, "Did the Messenger of Allah, peace and blessings of Allah be upon him, pray while sitting?" She said: "He used to pray for a long time at night while sitting, and he used to pray for a long time at night while standing. So when he recited while standing, he would bow while standing, and when he recited while sitting, he would bow while sitting."