Musannaf Ibn Abi Shayba 3:264

Waki’ narrated to us, from ‘Abbad bin Rashid, from al-Hasan, who said: Ahmar, the companion of the Messenger of Allah (may Allah bless him and grant him peace), narrated to me, saying: We used to feel pity for the Messenger of Allah (may Allah bless him and grant him peace) because of how he would keep his thighs away from his sides when he prostrated.