It is narrated from one of the Companions of the Messenger of Allah (peace and blessings be upon him) - most likely Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him) - that the Prophet (peace and blessings be upon him) commanded them to pray Zuhr in the cooler time, (and said:) "For the heat is from the exhalation of Hell."
Hadith Referenceالمعجم الكبير للطبراني / باب العين / 9757