Sahih Bukhari

Narated By Abu Dhar Al-Ghifar : We were with the Prophet on a journey and the Mu'adhdhin (call maker for the prayer) wanted to pronounce the Adhan (call) for the Zuhr prayer. The Prophet said, 'Let it become cooler." He again (after a while) wanted to pronounce the Adhan but the Prophet said to him, "Let it become cooler till we see the shadows of hillocks." The Prophet added, "The severity of heat is from the raging of the Hell-fire, and in very hot weather pray (Zuhr) when it becomes cooler."