Volume 1, Book 10, Number 511:
Narrated Abu Dhar:
The Muadhdhin (call-maker) of the Prophet pronounced the Adhan (call) for the Zuhr prayer but
the Prophet said, “Let it be cooler, let it be cooler.” Or said, ‘Wait, wait, because the severity of heat is
from the raging of the Hell-fire. In severe hot weather, pray when it becomes (a bit) cooler and the
shadows of hillocks appear.”