Sahih Bukhari | Volume 5 | Hadith Number 205
Narrated by '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."