Sahih Bukhari

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