Musnad Ahmad 27:830

'Abd al-Rahman narrated to us, Malik narrated to us, from al-Zuhri, from Salim, from Ibn 'Umar: "That the Messenger of Allah (peace and blessings of Allah be upon him) used to raise his hands when he began the prayer, and when he intended to bow, and when he raised his head from bowing, and he would not do that in prostration."