Musnad Ahmad 30:1754

Husayn ibn Muhammad narrated to us, he said: Ibn Abi Dhi'b informed us, from Muhammad ibn 'Amr ibn 'Ata', from Muhammad ibn 'Abd al-Rahman ibn Thawban, from Abu Hurayrah: "That the Prophet, may Allah bless him and grant him peace, when he stood—meaning for prayer—he would raise his hands, extending them."