Bukhari

Hadith Commentary: The mention of Umar (radi Allahu anhu) is also present through the narration of Sa’id bin Zayd (radi Allahu anhu); thus, there is a correspondence between the chapter and the hadith.

Sa’id (radi Allahu anhu) is expressing grief over the martyrdom of our master, Uthman al-Ghani (radi Allahu anhu), and is stating that this incident is so tremendous that if even the mountain of Uhud were to bear its impact, it would be appropriate to say: “Inna lillahi wa inna ilayhi raji’un.”

Indeed, the martyrdom of Uthman (radi Allahu anhu) was a very great calamity, from which discord began to enter Islam.

The incident of Umar (radi Allahu anhu) accepting Islam:
It is mentioned at length in the books of biography (seerah).

In summary, Abu Jahl said that whoever brings the head of Muhammad (sallallahu alayhi wa sallam), I will reward him with one hundred camels.

Umar (radi Allahu anhu) set out with his sword hanging by his side. On the way, someone said to him: “You can kill Muhammad (sallallahu alayhi wa sallam) later; first, understand your brother-in-law Sa’id bin Zayd (radi Allahu anhu) and your sister—they both have become Muslim.”

Umar (radi Allahu anhu) reached his sister’s house and tied up both his brother-in-law and his sister, beat them severely, and in the end, he became remorseful. He said to his sister: “Let me hear that speech which you and your husband were reciting when I arrived.”

She said: “You are without ablution (wudu); perform ablution.”

Umar (radi Allahu anhu) performed ablution, opened the mushaf, and began to read.

The effect was such that these pure words came from his tongue: “Ashhadu an la ilaha illa Allah wa ashhadu anna Muhammadan Rasul Allah.” Then he came to the Prophet (sallallahu alayhi wa sallam).

The Prophet (sallallahu alayhi wa sallam) said: “O Umar! Become Muslim.” He sincerely recited the testimony of faith, and all the Muslims proclaimed takbir in joy.

(Wahidi)

Allama Iqbal has described Umar’s (radi Allahu anhu) acceptance of Islam in this way: “You do not know that the burning passion of recitation changed the destiny of Umar”—that is, the burning passion in the recitation of the Noble Qur’an, which was manifest in the tone of his sister Fatimah (radi Allahu anha), changed the fate of Umar (radi Allahu anhu) and made him ready to accept Islam.

Alas, today the Noble Qur’an remains, and there are many who recite it, but that burning passion is missing.

The name of Umar’s (radi Allahu anhu) brother-in-law is Sa’id bin Zayd bin Amr bin Nufayl; he was also his paternal cousin.

The details have already been mentioned previously.

Hadith Commentary:
1.
Hazrat Sa’id bin Zayd radi Allahu anhu was the cousin and brother-in-law of Hazrat Umar radi Allahu anhu.
His sister, Hazrat Fatimah radi Allahu anha, was married to Hazrat Sa’id radi Allahu anhu.
When both of them accepted Islam, Hazrat Umar radi Allahu anhu, while still in a state of disbelief, used to tie them with ropes and humiliate them.
Hazrat Sa’id bin Zayd radi Allahu anhu intends to say that Hazrat Umar radi Allahu anhu treated us in such a manner while he was in a state of disbelief, whereas you people, despite being Muslims, have treated Hazrat Uthman radi Allahu anhu so badly that if, because of this, Mount Uhud were to collapse and shatter into pieces, it would indeed be deserving.
And Allah knows best.
2.
In the books of Seerah, the incident of Hazrat Umar radi Allahu anhu accepting Islam is written as follows: Abu Jahl announced:
Whoever brings the head of Muhammad sallallahu alayhi wa sallam, I will give him one hundred camels as a reward.
Hazrat Umar radi Allahu anhu set out with his sword drawn, when someone said:
First, take account of your brother-in-law and sister; both of them have become Muslim.
Hazrat Umar radi Allahu anhu turned towards his sister’s house.
Upon reaching there, he tied up his sister and brother-in-law with ropes and beat his sister severely.
Eventually, feeling ashamed, he said:
Let me hear the words you were reciting before I arrived.
When Hazrat Umar radi Allahu anhu heard and read those words, the testimony of faith (kalimah shahadah) spontaneously came from his mouth and he became Muslim.
With his acceptance of Islam, a wave of happiness spread among the Muslims.
Radi Allahu ta’ala anhu.