I would offer my noon prayer with the Messenger of Allah ﷺ and took a handful of gravels so that they might become cold in my hand and I placed them (before me) so that I may put my forehead on them at the time when I would prostrate. I did this due to the intensity of heat.  

كُنْتُ أُصَلِّي الظُّهْرَ مَعَ رَسُولِ اللَّهِ ﷺ فَآخُذُ قَبْضَةً مِنَ الْحَصَى لِتَبْرُدَ فِي كَفِّي أَضَعُهَا لِجَبْهَتِي أَسْجُدُ عَلَيْهَا لِشِدَّةِ الْحَرِّ  

The extent of the shadow when the Messenger of Allah prayed (the noon prayer) was three to five feet in summer and five to seven feet in winter.  

كَانَتْ قَدْرُ صَلاَةِ رَسُولِ اللَّهِ ﷺ فِي الصَّيْفِ ثَلاَثَةَ أَقْدَامٍ إِلَى خَمْسَةِ أَقْدَامٍ وَفِي الشِّتَاءِ خَمْسَةَ أَقْدَامٍ إِلَى سَبْعَةِ أَقْدَامٍ  

We were in the company of the Prophet ﷺ. The mu'adhdhin intended to call for the Zuhr prayer. He said: Make it cooler. He then intended to call for prayer. He said twice or thrice: Make it cooler. We then witnessed the shadow of the mounds. He then said: The intensity of heat comes from the bubbling over of the Hell ; so when the heat is violent, offer (the Zuhr) prayer when it becomes cooler.  

When the heat is violent, offer (the Zuhr) prayer when it becomes fairly cool, for the violent heat comes from the bubbling over the Hell.  

أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ إِذَا اشْتَدَّ الْحَرُّ فَأَبْرِدُوا عَنِ الصَّلاَةِ قَالَ ابْنُ مَوْهَبٍ بِالصَّلاَةِ فَإِنَّ شِدَّةَ الْحَرِّ مِنْ فَيْحِ جَهَنَّمَ  

Jabir b. Samurah reported that Bilal used to call for the noon prayer when the sun had declined.  

أَنَّ بِلاَلاً كَانَ يُؤَذِّنُ الظُّهْرَ إِذَا دَحَضَتِ الشَّمْسُ