Musnad Ahmad 316:4

Abu Nu‘aym narrated to us, Dawud—meaning Ibn Qays—narrated to us, he said: ‘Ubayd Allah ibn ‘Abd Allah ibn Aqram al-Khuza‘i narrated to me, he said: My father narrated to me that he was with his father at the plain of Namirah. He said: A mounted party passed by us and they made their camels kneel at the side of the road. My father said: "O my son, stay with your sheep until I go to these riders and ask them." He said: He approached them and I approached with him, and the prayer was called. Behold, the Messenger of Allah (peace and blessings of Allah be upon him) was among them. So I prayed with them, and it is as if I am looking at the whiteness of the armpits of the Messenger of Allah (peace and blessings of Allah be upon him) when he prostrated.