Sahih Muslim 619
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
Khabbab reported:
وَحَدَّثَنَا أَحْمَدُ بْنُ يُونُسَ ، وَعَوْنُ بْنُ سَلاَّمٍ ، - قَالَ عَوْنٌ أَخْبَرَنَا وَقَالَ ابْنُ يُونُسَ، وَاللَّفْظُ، لَهُ حَدَّثَنَا زُهَيْرٌ ، - قَالَ حَدَّثَنَا أَبُو إِسْحَاقَ ، عَنْ سَعِيدِ بْنِ وَهْبٍ ، عَنْ خَبَّابٍ ، قَالَ أَتَيْنَا رَسُولَ اللَّهِ صلى الله عليه وسلم فَشَكَوْنَا إِلَيْهِ حَرَّ الرَّمْضَاءِ فَلَمْ يُشْكِنَا . قَالَ زُهَيْرٌ قُلْتُ لأَبِي إِسْحَاقَ أَفِي الظُّهْرِ قَالَ نَعَمْ . قُلْتُ أَفِي تَعْجِيلِهَا قَالَ نَعَمْ .
We came to the Messenger of Allah (ﷺ) and we complained to the Messenger of Allah (ﷺ) about (saying prayer) on the extremely heated ground (or sand), but he paid no heed to us. Zuhair said: I asked Abu Ishaq whether it was about the noon prayer. He said: Yes. I again said whether it concerned the (offering) of the noon (prayer) in earlier hours. He said: Yes. I said: Did it concern expediting it? He said: Yes.