Sahih Bukhari
Times of the Prayers
Volume: 001 / Book Number: 010 / Hadith Number: 510
Narrated Abu Huraira and 'Abdullah bin 'Umar:
Allah's Apostle said, "If it is very hot, then pray the Zuhr prayer when it becomes (a bit) cooler, as the severity of the heat is from the raging of the Hell-fire."