(9) باب الإِبْرَادِ بِالظُّهْرِ فِي شِدَّةِ الْحَرِّ
Narrated Abu Dhar:
The Mu'adh-dhin (call-maker) of the Prophet (ﷺ) 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."
Reference: Sahih al-Bukhari 535
In-book reference: Book 9, Hadith 13
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 10, Hadith 511