I was with my father at the plain of Namirah when a group of riders passed by. There was the Messenger of Allah ﷺ standing in prayer. I could see the whiteness of his armpits whenever he prostrated.
كنت مع أبي بالقاع من نمرة، فمرت ركبة، فإذا رسول الله ﷺ قائم يصلي، قال: فكنت أنظر إلى عفرتي إبطيه إذا سجد، أي بياضه.
Tirmidhi (274), Nasai (1108), and Ibn Majah (881) all narrated it through the route of Dawud bin Qays, from Ubaydullah bin Abdullah al-Aqram, and the wording given is Tirmidhi's. Tirmidhi said, "Hasan. We do not know it except from the hadith of Dawud bin Qays, and we do not know of any hadith from Abdullah bin Aqram al-Khuzai, from the Prophet ﷺ, other than this one, and it is acted upon by most of the people of knowledge among the companions of the Prophet ﷺ." End of quote.I say, Dawud bin Qays is al-Farra al-Dabbagh, Abu Sulayman al-Qurashi, their freedman. Ahmad, Abu Zura, Nasai, and others deemed him trustworthy, so his being the sole narrator does not harm the hadith, and his teacher Ubaydullah bin Abdullah is also trustworthy. Hakim also graded it sahih (1/227).