Musnad Ahmad 1018:58

Muhammad ibn Ja‘far narrated to us, Shu‘bah narrated to us, he said: I heard Hajjaj ibn Hajjaj al-Aslami—and he was their Imam—narrating from his father, who used to perform Hajj with the Messenger of Allah (may Allah bless him and grant him peace), from a man among the companions of the Prophet (may Allah bless him and grant him peace)—Hajjaj said: I think it is ‘Abdullah—from the Prophet (may Allah bless him and grant him peace) that he said: "Indeed, the intensity of heat is from the scorching breath of Hell. So, when the heat intensifies, then delay the prayer until it cools."