Musnad Ahmad 937:243

Muhammad ibn Ja'far narrated to us, Shu'bah narrated to us, he said: I heard Muhajir Abu al-Hasan narrating that he heard Zayd ibn Wahb narrating from Abu Dharr, who said: The caller of the Messenger of Allah (peace and blessings of Allah be upon him) gave the adhan for Dhuhr. The Prophet (peace and blessings of Allah be upon him) said: "Cool down, cool down," or he said: "Wait, wait." And he said: "Indeed, the intensity of heat is from the scorching of Hell. So when the heat becomes intense, then cool down the prayer." Abu Dharr said: "Until we saw the shadows of the hillocks."