Jami` at-Tirmidhi 274
جامع الترمذي#274SahihUbaidallah bin Abdullah bin Arqam Al-Khuza'I narrated from his father (Abdullah bin Arqam), who said:"I was with my father at the plains of Namirah. I passed by a mount and saw Allah's Messenger standing in Salat." He said: "I was looking at the earthiness of his armpits when he prostrated." [That is]: "Whiteness
Scholarly Authentication
We show the most cautious grade (weakest-wins) as the overall label. Individual scholars graded it as follows:
Ahmad Muhammad ShakirSahih
Al-AlbaniSahih
Zubair Ali ZaiSahih
In-book reference: 2:126