Musnad Abu Ya'la al-Mawsili 129:76

Abu Bakr narrated to us, Abd al-Wahhab al-Thaqafi narrated to us, from Humayd, from Anas, who said: I saw the Messenger of Allah (may Allah bless him and grant him peace) raising his hands when he began the prayer, when he bowed, and when he raised his head from bowing.