Musnad Ahmad 937:88

'Affan narrated to us, Shu'bah narrated to us, from Muhajir Abu al-Hasan, who said: I heard Zayd ibn Wahb say: We came from a funeral and passed by Abu Dharr. He said: We were with the Messenger of Allah (peace and blessings of Allah be upon him) on a journey, and the mu'adhdhin wanted to give the adhan for Dhuhr. The Messenger of Allah (peace and blessings of Allah be upon him) said: "Let it cool." Then he wanted to give the adhan, and he said to him: "Let it cool." And the third time—to the best of my knowledge, Shu'bah said—he said to him [the same], until we saw the shadows of the hillocks. He said: "Indeed, the intensity of heat is from the scorching of Hell. So when the heat intensifies, delay the prayer until it cools."