Musannaf Ibn Abi Shayba 3:905

Shababa ibn Sawwar narrated to us, from Shu'ba, who said: Al-Muhajir Abu al-Hasan narrated to us, who said: I heard Zayd ibn Wahb narrating from Abu Dharr, who said: We were with the Messenger of Allah (may Allah bless him and grant him peace) on a journey, and Bilal wanted to give the adhan. The Messenger of Allah (may Allah bless him and grant him peace) said to him, "Cool down." Then he wanted to give the adhan, and he said, "Cool down," until we saw the shadow on the hillocks. Then he gave the adhan, and he prayed Dhuhr. Then he said, "Indeed, the intensity of heat is from the scorching of Hell. So when the heat intensifies, cool down with the prayer."