Musnad Abu Ya'la al-Mawsili 78:1

Abu Musa narrated to us, Abd al-Rahman ibn Mahdi narrated to us, Abbad ibn Rashid narrated to us, he said: I heard al-Hasan saying: Ahmar, a companion of the Prophet, may God bless him and grant him peace, narrated to us, he said: We used to feel pity for the Messenger of God, may God bless him and grant him peace, because of how he would keep his elbows away from his sides when he prostrated.