Sahih Bukhari Volume 008, Book 075, Hadith Number 392.

Narated By Abu Musa : The Prophet 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."