كتاب الْمَسَاجِدِ وَمَوَاضِعِ الصَّلاَةِ
5  
The Book of Mosques and Places of Prayer
(33)
Chapter: It is recommended to pray Zuhr at the beginning of its time when there is no intense heat
(33)
باب اسْتِحْبَابِ تَقْدِيمِ الظُّهْرِ فِي أَوَّلِ الْوَقْتِ فِي غَيْرِ شِدَّةِ الْحَرِّ ‏‏
Sahih Muslim 620

Anas b. Malik reported:

We used to say (the noonprayer) with the Messenger of Allah (ﷺ) in the intense heat, but when someone amongst us found it hard to place his forehead on the ground, he spread his cloth and prostrated on it.
حَدَّثَنَا يَحْيَى بْنُ يَحْيَى، حَدَّثَنَا بِشْرُ بْنُ الْمُفَضَّلِ، عَنْ غَالِبٍ الْقَطَّانِ، عَنْ بَكْرِ بْنِ عَبْدِ اللَّهِ، عَنْ أَنَسِ بْنِ مَالِكٍ، قَالَ كُنَّا نُصَلِّي مَعَ رَسُولِ اللَّهِ صلى الله عليه وسلم فِي شِدَّةِ الْحَرِّ فَإِذَا لَمْ يَسْتَطِعْ أَحَدُنَا أَنْ يُمَكِّنَ جَبْهَتَهُ مِنَ الأَرْضِ بَسَطَ ثَوْبَهُ فَسَجَدَ عَلَيْهِ ‏.‏
Reference : Sahih Muslim 620
In-book reference : Book 5, Hadith 241
USC-MSA web (English) reference : Book 4, Hadith 1296
  (deprecated numbering scheme)