Musnad Ahmad 658:202

Asbat narrated to us, Yazid ibn Abi Ziyad narrated to us, from Abd al-Rahman ibn Abi Layla, from al-Bara’ ibn Azib, who said: "When the Messenger of Allah, peace and blessings of Allah be upon him, would begin the prayer, he would raise his hands until his thumbs were level with his ears."