Musnad Abu Ya'la al-Mawsili 143:6

Sarij ibn Yunus narrated to us, Bishr ibn al-Mufaddal narrated to us, from Ghalib, from Bakr ibn Abdullah, from Anas, who said: We used to pray with the Messenger of Allah (peace and blessings of Allah be upon him) in the intense heat. One of us would take pebbles in his hand, and when they cooled down, he would place them and prostrate upon them.