كتاب الْمَسَاجِدِ وَمَوَاضِعِ الصَّلاَةِ
5  
The Book of Mosques and Places of Prayer
(32)
Chapter: The recommendation to delay Zuhr (until it has cooled down) in severe heat for one who finds it too hot in route to join the congregation
(32)
باب اسْتِحْبَابِ الإِبْرَادِ بِالظُّهْرِ فِي شِدَّةِ الْحَرِّ لِمَنْ يَمْضِي إِلَى جَمَاعَةٍ وَيَنَالُهُ الْحَرُّ فِي طَرِيقِهِ
Sahih Muslim 615 d

Abu Huraira reported that the Messenger of Allah (ﷺ) said:

This heat is from the exhalation of Hell-fire, so delay the prayer till it is cool.
وَحَدَّثَنَا قُتَيْبَةُ بْنُ سَعِيدٍ، حَدَّثَنَا عَبْدُ الْعَزِيزِ، عَنِ الْعَلاَءِ، عَنْ أَبِيهِ، عَنْ أَبِي هُرَيْرَةَ، أَنَّ رَسُولَ اللَّهِ صلى الله عليه وسلم قَالَ ‏ "‏ إِنَّ هَذَا الْحَرَّ مِنْ فَيْحِ جَهَنَّمَ فَأَبْرِدُوا بِالصَّلاَةِ ‏"‏ ‏.‏
Reference : Sahih Muslim 615d
In-book reference : Book 5, Hadith 232
USC-MSA web (English) reference : Book 4, Hadith 1287
  (deprecated numbering scheme)