Hazrat ‘Umar (radhiyallahu ‘anhu) embraced Islam, and almost immediately, the honor and dominance of Islam began to increase. The Muslims started performing salaah publicly in the Haram, and they began inviting towards Islam openly. From that day, the distinction between truth and falsehood became clear and apparent. It is for this reason that Rasulullah (sallallahu ‘alaihi wasallam) gave Hazrat ‘Umar (radhiyallahu ‘anhu) the title “Al-Faarooq” - the one who distinguishes between truth and falsehood.
Hazrat ‘Umar (radhiyallahu ‘anhu) relates:
When I embraced Islam, I thought to myself that I should inform such a person of my conversion to Islam who is excellent in publicizing information. In this way, all the people will become aware of my Islam. Accordingly, I went to Jameel bin Ma‘mar who was well known for publicizing information. I said to him, “O Jameel! Are you aware that I have become a Muslim and have embraced the religion of Muhammed (sallallahu ‘alaihi wasallam)?”
The moment Jameel heard this, he ran to the Haram, dragging his shawl in haste. The leaders of the Quraish were assembled in the Haram, and on reaching them, he called out, “O people! ‘Umar has turned Saabi (apostate)!” I was right behind him and exclaimed, “He is incorrect! I am not a Saabi, rather I have embraced Islam. I testify that there is none worthy of worship but Allah and that Muhammed (sallallahu ‘alaihi wasallam) is His slave and messenger.”
As soon as the people heard this, they fell on Hazrat ‘Umar (radhiyallahu ‘anhu), beating him and hitting him. This continued for the whole day. .
At that time, ‘Aas bin Waa’il Sahmi happened to pass by. He asked the people, “What is the matter?” They replied, “’Umar has become a Saabi.” ‘Aas responded, “So what? A person has the liberty to choose whatever religion he wishes (in other words, why are you opposing him in his personal decision?) If you ever kill him, do you think that his clan, the Banu ‘Adi, will leave you and not take revenge? Leave him and go, for I have granted amnesty to him.”
After ‘Aas granted amnesty to Hazrat ‘Umar (radhiyallahu ‘anhu), the crowd of people left him and dispersed.
(Extracted from Seeratul Mustafa 1/262-263)