Sahih Bukhari Volume 1, Book 10, Hadith Number 511

Narated By Abu Dhar : The Muadhdhin (call-maker) of the Prophet pronounced the Adhan (call) for the Zuhr prayer but the Prophet said, “Let it be cooler, let it be cooler.” Or said, ‘Wait, wait, because the severity of heat is from the raging of the Hell-fire. In severe hot weather, pray when it becomes (a bit) cooler and the shadows of hillocks appear.”