The most notable event of the year 6 of the Call was the conversion to Islam of Umar ibn al-Khattab, a future khalifa of the Muslims. He was one of the most rabid enemies of Islam and Muhammed Mustafa, the Messenger of Allah, and was a great tormentor of the Muslims. The modern Egyptian historian, Amin Dawidar, says in his book, Pictures From the Life of the Prophet, that Umar's hatred of Islam, and his hostility to Muhammed Mustafa, were matched only by the hatred of and hostility to them, of his own maternal uncle, Abu Jahl.
It is said that one day Umar resolved, in sheer exasperation, to kill Muhammed Mustafa, and thereby to extinguish the flame of Islam itself. He left his home with this intention.
As noted before, the Muslims at this time (the end of the year 6) still gathered in the house of Arqam ibn Abil Arqam to say their congregational prayers. They were just beginning to assemble, when one of them, looking out the window, saw Umar approaching the house with a drawn sword. In a state of considerable alarm, he told 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; 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 the Prophet when a passer-by stopped him, and informed him that his own sister and her husband had become Muslims; and advised him to put his own house in order before undertaking any other grandiose and chimerical project.

Muhammad Husayn Haykal

Umar went there (to Dar-ul-Arqam) resolved to kill Muhammad and thus relieve the Quraysh of its burden, restore its ravaged unity, and re-establish respect for the gods that Muhammad had castigated. On the road to Makkah he was met by Nu'aym ibn Abdullah. Upon learning what Umar was about, Nu'aym said, "By God, you have deceived yourself, O Umar! Do you think that Banu Abd Manaf would let you run around alive once you kill Muhammad. Your sister is a Muslim now. Why don't you return to your own house and set it straight?"
(The Life of Muhammad, Cairo, 1935)
Umar was furious to hear this. He immediately changed his direction from the house of Arqam to the house of his sister to investigate the allegation. In reply to his question, she gave him a discreet but evasive answer.

Muhammad ibn Ishaq

Umar came to the door (of the house of his sister) as Khabbab (a companion of the Prophet) was studying the Sura Taha and When the Sun is Overthrown. The pagans 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...
(The Life of the Messenger of God)
Umar exploded in wrath at what he thought to be a prevarication, and struck his sister in her face. The blow caused her mouth to bleed. Umar 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 Allah."
Umar immediately went home, washed himself, returned to his sister's house, read the text of Quran, and then went to Arqam's house where he bore witness to the Unity of the Creator and the Prophethood of Muhammed.
Sir William Muir says that Umar's conversion to Islam took place at the close of the sixth year of Mohammed's mission.

Sir William Muir

It (Umar's conversion) occurred in Dzul Hajj, the last month of the year. The believers are said now to have amounted in all to 40 men and ten women; or by other accounts, to 45 men and eleven women. (The Life of Mohammed, London, 1877) Umar was in his thirties when he became a Muslim.

Muhammad Husayn Haykal

At that time, Umar ibn al-Khattab was a mature man of thirty to thirty-five years of age. (The Life of Muhammad, Cairo, 1935)