Musnad Abu Ya'la al-Mawsili 150:224

'Abd al-A'la narrated to us, Wahib ibn Khalid narrated to us, Humayd narrated to us, from 'Abdullah ibn Shaqiq, from 'A'ishah, that the Messenger of Allah (may Allah bless him and grant him peace) used to pray standing and sitting. When he prayed standing, he would bow standing, and when he prayed sitting, he would bow sitting.