He was with his father at al-Qa from Namirah when a group of riders passed by us. His father said, "My son, stay with your young livestock while I go to these people and ask them some questions."He approached them, and I drew near as well. I could see the whiteness of the underarms of the Messenger of Allah ﷺ as he was prostrating.
حدثنا عبد الرحمن بن مهدي قال حدثنا داود بن قيس
أنه كان مع أبيه بالقاع من نمرة فمر بنا ركب فقال أبي يا بني كن في بهمك حتى آتي هؤلاء القوم فأسائلهم فدنا ودنوت فكنت أنظر إلى عفرتي إبطي رسول الله ﷺ وهو ساجد
Its isnad is sahih. Ubaydullah bin Abdullah bin Arqam was from Hijaz and reliable, Ahmad deemed him reliable, and his hadith appears in the Sunan works. The hadith was narrated by Nasai 2/213 no. 1108, and Ibn Majah 1/285 no. 881, and it has already appeared at 12794.