Musannaf Ibn Abi Shayba 3:903

Abu Bakr narrated to us, he said: Abu Mu'awiyah narrated to us, from al-A'mash, from Abu Sa'id al-Khudri, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Perform the prayer when it is cool—meaning the Dhuhr prayer—for the intensity of the heat is from the scorching of Hell."