When the Prophet ﷺ finished with Hunayn, he sent Abu Amir at the head of an army to Awtas. The hadith continues, and includes the following, Then he raised his hands and said, "O Allah, forgive Ubayd Abu Amir." And I saw the whiteness of his armpits.
لما فرغ النبي ﷺ من حنين، بعث أبا عامر على جيش إلى أوطاس. الحديث وفيه: ثم رفع يديه فقال: اللهم اغفر لعبيد أبي عامر. ورأيت بياض إبطيه.
Bukhari related it in the Book of Supplications (6323), and Muslim in the Merits of the Companions (2498), both of them from Abu Kurayb Muhammad bin Ala, who said, Abu Usama narrated to us, from Burayd bin Abdullah, from Abu Burda, from his father Abu Musa Ashari, who said, and then he mentioned it.