Sahih Muslim
The Book of Prayers (Kitab Al-Salat)
Book Number: 004 / Hadith Number: 1296
Anas b. Malik reported: We used to say (the noonprayer) with the Messenger of Allah (may peace be upon him) in the intense heat, but when someone amongst us found it hard to place his forehead on the ground, he spread his cloth and prostrated on it.