Abu Dharr narrated:"Allah's Messenger was on a journey and Bilal was with him. So he wanted to call for the prayer, but he (the Prophet) said: 'Let it get cooler.' Then he wanted to call for the prayer, so Allah's Messenger said: 'Let it get to the cooler time of Zuhr.'" He (i.e., Abu Dharr) said: "Until we saw the shadows of the hillocks, then he commanded that the Iqamah be called and then led the people in prayer. Allah's Messenger said: 'The severity of heat is from the raging of Hell, so wait until it becomes cooler for the (Zuhr) prayer

English translation · Jami' at-Tirmidhi