Bukhari Collection: ::Book Number 5:: Hadith # 205
Narrated `Abdullah bin `Umar:When `Umar embraced Islam, all The (disbelieving) people gathered around his home and said, "`Umar has embraced Islam." At that time I was still a boy and was on the roof of my house. There came a man wearing a cloak of Dibaj (i.e. a kind of silk), and said, "`Umar has embraced Islam. Nobody can harm him for I am his protector." I then saw the people going away from `Umar and asked who the man was, and they said, "Al-`As bin Wail."