Maliks Muwatta Book 1, Hadith Number 29.
Section : Prohibition against Doing the Prayer at the Hottest Hour of the Day.
Yahya related to me from Malik from Abu’z Zinad from al-Araj from Abu Hurayra that the Messenger of Allah, may Allah bless him and grant him peace, said, “When the heat is fierce, wait until it gets cooler before you do the prayer, for scorching heat is from the blast of Jahannam.”