Musnad Abu Ya'la al-Mawsili 108:149

Abu Khaythama narrated to us, 'Abbad ibn al-'Awwam narrated to us, Muhammad ibn 'Amr informed us, from Sa'id ibn al-Harith, from Jabir ibn 'Abd Allah, who said: I used to pray the Dhuhr prayer with the Messenger of Allah, peace and blessings of Allah be upon him, and I would take a handful of pebbles and place them in my palm, then transfer them to the other palm until they cooled down. Then I would place them for my forehead when I prostrated, due to the intensity of the heat.