Musnad Ahmad 316:3

Waki’ narrated to us, he said: Dawud ibn Qays narrated to us, from ‘Ubayd Allah ibn ‘Abd Allah ibn Aqram al-Khuza’i, from his father, who said: I was with my father, Aqram, at the plain. He said: A caravan passed by us and they camped at the side of the road. My father said to me: "O my son, stay with your sheep until I go to these people and ask them." He said: So he went out, and I went out following in his footsteps, and behold, it was the Messenger of Allah (peace and blessings of Allah be upon him). He said: Then the prayer time arrived, so I prayed with him, and I was looking at the whiteness of the armpits of the Messenger of Allah (peace and blessings of Allah be upon him) whenever he prostrated.