Musnad Ahmad 1153:1670

Waki' narrated to us, he said: Yazid—meaning Ibn Ibrahim—narrated to us, from Ibn Sirin, from Abdullah ibn Shaqiq, from Aisha, who said: The Messenger of Allah (peace and blessings of Allah be upon him) used to pray standing and sitting. If he began the prayer standing, he would bow standing. And if he began the prayer sitting, he would bow sitting.