Musnad Ahmad 316:2

'Abd al-Rahman ibn Mahdi narrated to us, he said: Dawud ibn Qays narrated to us, from 'Ubayd Allah ibn 'Abd Allah ibn Aqram, he said: My father narrated to me that he was with his father at the plain of Namirah. A caravan passed by us, and my father said: "O my son, stay with your sheep until I go to these people and ask them." He drew near, and I drew near. I was looking at the whiteness of the armpits of the Messenger of Allah, may Allah bless him and grant him peace, while he was prostrating.