Musnad Ahmad 645:2

Ya'la narrated to us, Abu Isma'il—meaning Bashir—narrated to us, from Al-Qasim ibn Safwan al-Zuhri, from his father, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Delay the Dhuhr prayer until it is cooler, for the intensity of the heat is from the surging of Hell."