(10) باب الإِبْرَادِ بِالظُّهْرِ فِي السَّفَرِ
Narrated Abu Dhar Al-Ghifar:
We were with the Prophet (ﷺ) on a journey and the Mu'adh-dhin (call maker for the prayer) wanted to pronounce the Adhan (call) for the Zuhr prayer. The Prophet (ﷺ) said, 'Let it become cooler." He again (after a while) wanted to pronounce the Adhan but the Prophet (ﷺ) said to him, "Let it become cooler till we see the shadows of hillocks." The Prophet (ﷺ) added, "The severity of heat is from the raging of the Hell-fire, and in very hot weather pray (Zuhr) when it becomes cooler."
Reference: Sahih al-Bukhari 539
In-book reference: Book 9, Hadith 16
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 1, Book 10, Hadith 514