Prayer (Kitab Al-Salat)
Dawud :: Book 2 : Hadith 399
Narrated Jabir ibn Abdullah:
I would offer my noon prayer with the Apostle of Allah (peace_be_upon_him) and took a handful of gravels so that they might become cold in my hand and I placed them (before me) so that I may put my forehead on them at the time when I would prostrate. I did this due to the intensity of heat.