Musnad Ahmad 937:152

Hajjaj narrated to us, Shu'bah said: We were informed by Muhajir Abu al-Hasan, from the tribe of Taym Allah, a freed slave of theirs, who said: We returned from a funeral and passed by Zayd ibn Wahb, and he narrated from Abu Dharr, who said: We were with the Prophet, peace and blessings of Allah be upon him, on a journey. The caller to prayer (mu'adhdhin) wanted to give the adhan, so the Prophet, peace and blessings of Allah be upon him, said: "Cool it down." Then he wanted to give the adhan, and the Prophet, peace and blessings of Allah be upon him, said: "Cool it down." He said it three times. He said: Until we saw the shadows of the hillocks, then he prayed. Then he said: "Indeed, the intensity of heat is from the scorching of Hell. So when the heat is intense, cool down the prayer."