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."
حَدَّثَنَا عَلِيُّ بْنُ عَبْدِ اللَّهِ، حَدَّثَنَا سُفْيَانُ، قَالَ عَمْرُو بْنُ دِينَارٍ سَمِعْتُهُ قَالَ قَالَ عَبْدُ اللَّهِ بْنُ عُمَرَ ـ رضى الله عنهما ـ لَمَّا أَسْلَمَ عُمَرُ اجْتَمَعَ النَّاسُ عِنْدَ دَارِهِ وَقَالُوا صَبَا عُمَرُ. وَأَنَا غُلاَمٌ فَوْقَ ظَهْرِ بَيْتِي، فَجَاءَ رَجُلٌ عَلَيْهِ قَبَاءٌ مِنْ دِيبَاجٍ فَقَالَ قَدْ صَبَا عُمَرُ. فَمَا ذَاكَ فَأَنَا لَهُ جَارٌ. قَالَ فَرَأَيْتُ النَّاسَ تَصَدَّعُوا عَنْهُ فَقُلْتُ مَنْ هَذَا قَالُوا الْعَاصِ بْنُ وَائِلٍ.
Bukhari
Merits of the Helpers in Madinah (Ansaar) • Hadith 90