Musnad Ahmad 30:1779

Aswad ibn Amir narrated to us, Abu Bakr informed us, from Asim, from Abu Salih, from Abu Hurairah, 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 its heat is from the scorching of Hell."