Musnad Abu Ya'la al-Mawsili 42:336

Zuhayr narrated to us, Muhammad bin Ubayd narrated to us, from al-A'mash, from Abu Salih, from Abu Sa'id, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Delay the Dhuhr prayer during the heat, for the intensity of the heat is from the scorching of Hell."