Musnad Ahmad 30:3671

Yahya ibn Adam narrated to us, Abu Bakr ibn Ayyash narrated to us, from Asim ibn Abi al-Najud, from Abu Salih, from Abu Hurayrah, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Delay the Dhuhr prayer until it cools down in the heat, for its heat is from the scorching of Hell."