Sahih al-Bukhari Book 9, Hadith 15
that Allah's Messenger (ﷺ) said, "Pray Zuhr prayer when it becomes (a bit) cooler as the severity of heat is
from the raging of the Hell-fire."
The Prophet (ﷺ) said, "In very hot weather delay the Zuhr prayer till it becomes (a bit) cooler because the severity of heat is from the raging of the Hell-fire. The Hell-fire of Hell complained to its Lord saying: O Lord! My parts are eating (destroying) one another. So Allah allowed it to take two breaths, one in the winter and the other in the summer. The breath in the summer is at the time when you feel the severest heat and the breath in the winter is at the time when you feel the severest cold."
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."