#Subhanallah 
Umar's Conversion to Islam - A.D. 616
The most notable event of the year 6 of the Proclamation was the conversion to Islam of Umar bin al-Khattab, a future khalifa of the Muslims. He was one of the most rabid enemies of Islam and of Muhammad     , the Messenger of God, and was a great tormentor of the Muslims. The modern Egyptian historian, Amin Dawidar, says that Umar's hatred of Islam, and his hostility to Muhammad    , were matched only by the hatred of, and hostility to them, of his own maternal uncle, Abu Jahl.

It is said that one day in sheer exasperation, Umar resolved to kill Muhammad, and thus to extinguish the flame of Islam itself. He left his home with this intention.

As already noted, the Muslims at this time (the last days of the year 6) still gathered in the house of Arqam bin Abi al-Arqam to say their congregational prayers. They were beginning to assemble when one of them, looking out the window, saw Umar approaching toward the house with a drawn sword. In a state of considerable alarm, he told the other members of the congregation what he saw. Presumably, they too were alarmed.

But Hamza, who was also present in the house of Arqam, reassured them, and said that if Umar was coming with good intentions, then it was all right; but if not, then he (Hamza) would run him (Umar) through with his (Umar's) own sword. But it so happened that Umar had come with the intention of accepting Islam, and he did.

The story is told that Umar was going toward Dar-ul-Arqam with the intention of killing Muhammad  when a passer-by stopped him, and informed him that his own sister and her husband had become Muslims, and advised him to  return to is own house and at least set it straight .

Umar was furious to hear that his sister and her husband had become Muslims. He immediately changed his direction from Arqam's house to her house to investigate the allegation. In reply to his questions, she gave a discreet but evasive answer.

Umar came to the door (of the house of his sister) as Khabbab (a companion of the Prophet) was studying under her guidance the Sura Taha and also “When the Sun is Overthrown” (81:1). The polytheists used to call this reading “rubbish”.

When Umar came in, his sister saw that he meant mischief and hid the sheets from which they were reading. Khabbab slipped away into the house. Umar asked what was the gibberish he had heard, to which she answered that it was merely conversation between them...” 

Umar exploded in wrath at what he believed to be a prevarication, and struck his sister in her face. The blow caused her mouth to bleed. He was going to strike again but the sight of blood made him pause. He suddenly appeared to relent, and then in a changed tone asked her to show him what she was reading. She sensed a change in him but said: “You are an unclean idolater, and I cannot allow you to touch the Word of God.”

Umar immediately went away, washed himself, returned to his sister's home, read the text of Qur'an, and then went to the house of Arqam where he formally accepted Islam.