From Sahih Bukhari (Hadith 6383):
Narrated Abu Musa:The Prophet (ﷺ) asked for some water and performed the ablution, and then raised his hands (towards the sky) and said, "O Allah! Forgive `Ubaid Abi 'Amir." I saw the whiteness of his armpits (while he was raising his hands) and he added, "O Allah! Upgrade him over many of Your human creatures on the Day of Resurrection
Reference: Bukhari 6383