Muwatta Malik #27

Yahya related to me from Malik from Zayd ibn Aslam from Ata ibn Yasar that the Messenger of Allah, may Allah bless him and grant him peace, said, "Scorching heat is a part of the blast of Jahannam. So, when the heat is fierce, delay the prayer until it gets cooler." He added in explanation, "The Fire complained to its Lord and said, 'My Lord, part of me has eaten another part,' so He allowed it two breaths in every year, a breath in winter and a breath in summer."