Musannaf Ibn Abi Shayba 3:265

Waki’ narrated to us, from Dawud ibn Qays, from ‘Abdullah ibn ‘Abdullah ibn Aqram al-Khuza’i, from his father, who said: I was with my father at the plain of Namirah, and a caravan passed by us and they made their camels kneel at the side of the road. He said: "O my son, stay with your sheep until I go to these people." So he went out, and I went out with him. He approached, and I approached, and behold, it was the Messenger of Allah (ﷺ). He prayed, and I prayed with him, and I was looking at the whiteness of his armpits.