Musannaf Ibn Abi Shayba 3:898

Abbad ibn al-Awwam narrated to us, from Muhammad ibn Amr, from Sa'id ibn al-Harith, from Jabir ibn Abdullah, who said: I was praying the Dhuhr prayer with the Messenger of Allah (may Allah bless him and grant him peace), so I would take a handful of pebbles and place them in my palm. Then I would transfer them to the other palm until they cooled down, and then I would place them for my forehead when I prostrated, due to the intensity of the heat.