Musnad Ahmad 32:30

Bishr ibn al-Mufaddal narrated to us, Ghalib al-Qattan narrated to us, from Bakr ibn Abdullah, from Anas ibn Malik, who said: "We used to pray with the Prophet, peace and blessings of Allah be upon him, in intense heat. If one of us could not place his face firmly upon the ground, he would spread his garment and prostrate upon it."