Sahih Muslim 620
Sahih Muslim · Book 5: The Book of Mosques and Places of Prayer · It is recommended to pray Zuhr at the beginning of its time when there is no intense heat
Anas b. Malik reported:
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 .
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.