Musnad Ahmad 33:392

Khalaf ibn al-Walid narrated to us, 'Abbad ibn 'Abbad narrated to us, from Muhammad ibn 'Amr, from Sa'id ibn al-Harith al-Ansari, from Jabir ibn 'Abdullah, 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 in my palm so that they would cool down, until I would prostrate upon them due to the intensity of the heat."