Musnad Ahmad 1153:2271

'Abd al-Wahhab narrated to us, from Sa'id, from Budayl ibn Maysarah, from 'Abdullah ibn Shaqiq, from 'Aishah: "The Prophet, peace and blessings of Allah be upon him, used to pray much while standing and while sitting. So when he prayed sitting, he would bow while sitting, and when he prayed standing, he would bow while standing."