Musnad Ahmad 27:1703

Al-Hakam ibn Nafi' narrated to us, Isma'il ibn 'Ayyash narrated to us, from Salih ibn Kaysan, from 'Abd al-Rahman al-A'raj, from Abu Hurayrah: "That the Prophet, peace and blessings of Allah be upon him, used to raise his hands level with his shoulders when he said the takbir and began the prayer, and when he bowed, and when he prostrated."