hadith

 Sahih Bukhari  صحیح بخاری

Hadith # 535

Translation:

Narrated Abu Dhar (RA): The Muadhdhin (call-maker) of the Prophet ﷺ pronounced the Adhan (call) for the Zuhr prayer but the Prophet ﷺ 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."