Musnad Ahmad 658:210

Asbat ibn Muhammad 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, may Allah bless him and grant him peace, would begin the prayer, he would raise his hands until his thumbs were level with his ears."