Haafiz Ibnu ‘Asaakir (rahimahullah) mentions that Rasulullah (sallallahu ‘alaihi wasallam) was thereafter informed via wahi that Abu Jahal would not embrace Islam. Thus, Rasulullah (sallallahu ‘alaihi wasallam) then began to make du‘aa exclusively for Hazrat ‘Umar (radhiyallahu ‘anhu) saying:
اللهم أيد الإسلام بعمر بن الخطاب خاصةً
“O Allah! Strengthen Islam with ‘Umar bin Khattaab in particular.”
In essence, the actual cause of Hazrat ‘Umar (radhiyallahu ‘anhu) accepting Islam was the effect of the du‘aa of Rasulullah (sallallahu ‘alaihi wasallam).
As for the apparent cause of his conversion, then Hazrat ‘Umar (radhiyallahu ‘anhu) himself narrates the manner in which he accepted Islam. Hazrat ‘Umar (radhiyallahu ‘anhu) says:
“Initially, I was an arch enemy of Rasulullah (sallallahu ‘alaihi wasallam) and I harbored deep hatred for Islam. One day, Abu Jahal made an announcement saying, “I stand guarantee to give the prize of one hundred camels to the person who kills Muhammed (sallallahu ‘alaihi wasallam).” When I heard of the announcement, I went to Abu Jahal and personally asked him as to whether the announcement I had heard was true. Abu Jahal confirmed to me that the announcement I had heard was correct.
“I thus took my sword and set out with the intention of killing Muhammed (sallallahu ‘alaihi wasallam). While I was walking to fulfil my intention, I saw a calf which some people were about to slaughter. Seeing this, I paused and stood to watch them slaughter the animal. As I watched, I suddenly heard a voice coming out from the stomach of the calf. The voice proclaimed:
يا آل ذريح أمر نجيح رجل يصيح بلسان فصيح يدعو إلى شهادة أن لا إله إلا الله وأن محمدا رسول الله
“O family of Zareeh! This is a matter that pertains to one acquiring eternal success! There is a man who calls out in eloquent speech. He invites people to bear witness that there is none worthy of worship besides Allah, and that Muhammed (sallallahu ‘alaihi wasallam) is the Rasul of Allah”
Hazrat ‘Umar (radhiyallahu ‘anhu) says, “On hearing this voice, I immediately realized that this voice was being directed towards me and was addressing me.”
Despite hearing the voice from the stomach of the calf, Hazrat ‘Umar (radhiyallahu ‘anhu) did not change his mind and still continued to fulfil his sinister intention of killing Rasulullah (sallallahu ‘alaihi wasallam). Hence, he left the scene, and on taking just a few steps, he met Hazrat Nu‘aim bin ‘Abdillah Nahhaam (radhiyallahu ‘anhu).
Hazrat Nu‘aim (radhiyallahu ‘anhu) asked him, “O ‘Umar! Where are you off to at this time of the afternoon?” Hazrat ‘Umar (radhiyallahu ‘anhu) replied, “I am on my way to kill Muhammed (sallallahu ‘alaihi wasallam).” Hazrat Nu‘aim (radhiyallahu ‘anhu) remarked, “After killing Muhammed (sallallahu ‘alaihi wasallam), how do you expect to protect yourself from the clans of Banu Haashim and Banu Zuhrah?”
Hearing this, Hazrat ‘Umar (radhiyallahu ‘anhu) became angered and responded, “It seems as though you have also turned Saabi (apostate) and have renounced the religion of your ancestors!” Hazrat Nu‘aim (radhiyallahu ‘anhu) replied, “Why are you criticizing me for accepting Islam? Do you not know that your own sister, Hazrat Faatimah bint Khattaab (radhiyallahu ‘anha), and your brother-in-law, Hazrat Sa‘eed bin Zaid (radhiyallahu ‘anhu), have also turned Saabi? Have you not heard that they have also renounced your ancestral religion in favor of Islam?”
When Hazrat ‘Umar (radhiyallahu ‘anhu) heard this, he became infuriated and immediately set out for his sister’s home. At that time, Hazrat Khabbaab (radhiyallahu ‘anhu) was in her house, engaged in teaching the Qur’aan to her and her husband. However, as soon as he heard the footsteps of Hazrat ‘Umar (radhiyallahu ‘anhu), he concealed himself and hid from Hazrat ‘Umar (radhiyallahu ‘anhu).
Hazrat ‘Umar (radhiyallahu ‘anhu) entered the house and addressed his sister and brother-in-law saying, “Perhaps the two of you have also become apostates?” His brother-in-law replied, “O ‘Umar! Tell me, if your religion is not the true religion, while another religion is the true religion, then what should one do?” The moment he said this, Hazrat ‘Umar (radhiyallahu ‘anhu) began to beat him. His sister tried to intervene to protect her husband, but he also struck her so badly that her face began to bleed.
His sister then said to him, “O son of Khattaab! Do whatever you wish, for we have embraced Islam. O enemy of Allah! Are you beating us merely because we believe in the Oneness of Allah? Understand that we have embraced Islam, whether you like it or not.”
On hearing these words from his sister, Hazrat ‘Umar (radhiyallahu ‘anhu) felt ashamed and said, “Very well! In that case, show me the book that you were reciting from a few moments ago.” The instant Hazrat ‘Umar (radhiyallahu ‘anhu) said this, Hazrat Khabbaab (radhiyallahu ‘anhu), who was hiding in a corner of the house, suddenly emerged.
The sister of Hazrat ‘Umar (radhiyallahu ‘anhu) then said to Hazrat ‘Umar (radhiyallahu ‘anhu):
إنك رجس وإنه لا يمسه إلا المطهرون فقم فتوضأ
“You are in a state of impurity and hence you are unfit to touch the Qur’aan. Only those in the state of purity may touch the Qur’aan. Thus, you should go and purify yourself.”
Hazrat ‘Umar (radhiyallahu ‘anhu) stood up and then performed either wudhu or ghusl. He then picked up the pages, in which Surah Taahaa was written, and began to recite, until he reached the following verse:
اِنَّنِیۡۤ اَنَا اللّٰہُ لَاۤ اِلٰہَ اِلَّاۤ اَنَا فَاعۡبُدۡنِیۡ ۙ وَ اَقِمِ الصَّلٰوۃَ لِذِکۡرِیۡ ﴿۱۴﴾
“Certainly, I am Allah! There is none worthy of worship besides Me. So worship Me and establish salaah for My remembrance.” (Surah Taahaa v. 14)
The moment he recited this verse, he spontaneously exclaimed, “How beautiful and noble are these words!”
Hearing Hazrat ‘Umar (radhiyallahu ‘anhu) say this, Hazrat Khabbaab (radhiyallahu ‘anhu) remarked, “O ‘Umar! Glad tidings to you! I have hope that the du‘aa which Rasulullah (sallallahu ‘alaihi wasallam) made for you has been accepted.” Hazrat ‘Umar (radhiyallahu ‘anhu) then said, “O Khabbaab! Take me to Rasulullah (sallallahu ‘alaihi wasallam).”
Hazrat Khabbaab (radhiyallahu ‘anhu) agreed and took Hazrat ‘Umar (radhiyallahu ‘anhu) to Daar-ul-Arqam where Rasulullah (sallallahu ‘alaihi wasallam) and the Sahaabah (radhiyallahu ‘anhum) would congregate. The door was closed, so he knocked on the door and requested permission to enter.
When the Sahaabah (radhiyallahu ‘anhum) in the home heard that it was Hazrat ‘Umar (radhiyallahu ‘anhu) who wished to enter, then none of them dared to open the door. However, Hazrat Hamzah (radhiyallahu ‘anhu) said, “Open the door and allow him to enter. If Allah Ta‘ala wishes goodness for ‘Umar, then Allah will bless him with guidance. He will embrace Islam and follow the Rasul of Allah (sallallahu ‘alaihi wasallam). If he intends otherwise, then by the command of Allah Ta‘ala, you will be safe from his evil. All praises to Allah - it is not difficult for us to put ‘Umar to the sword.”
According to another narration, Hazrat Hamzah (radhiyallahu ‘anhu) said. “If ‘Umar comes to us with a good intention, we will also deal with him favorably, but if he has come here with an evil intention, then we will kill him with his own sword.” Rasulullah (sallallahu ‘alaihi wasallam) also permitted the Sahaabah (radhiyallahu ‘anhum) to open the door.
Hazrat ‘Umar (radhiyallahu ‘anhu) narrates: “Once the door was opened, two people held my arms and brought me before Rasulullah (sallallahu ‘alaihi wasallam). Rasulullah (sallallahu ‘alaihi wasallam) instructed them to release me, and held my kurtah and pulled me towards him saying, “O son of Khattaab! Accept Islam!” Saying this, Rasulullah (sallallahu ‘alaihi wasallam) made the following du‘aa:
اللّهم اهده
“O Allah! Bless ‘Umar with Your divine guidance!”
According to another narration, Rasulullah (sallallahu ‘alaihi wasallam) made the following du‘aa:
اللّهم هذا عمر بن الخطاب اللّهم أعز الدين بعمر بن الخطاب
“O Allah! This is ‘Umar bin Khattaab before You! O Allah! Bring honor to Deen (and the Muslims) through ‘Umar bin Khattaab!”
Rasulullah (sallallahu ‘alaihi wasallam) then addressed Hazrat ‘Umar (radhiyallahu ‘anhu) saying, “O ‘Umar! Will you only cease when Allah Ta‘ala sends down a humiliating punishment upon you?” Hazrat ‘Umar (radhiyallahu ‘anhu) replied, “O Rasul of Allah (sallallahu ‘alaihi wasallam)! I have come here to bring Imaan in Allah, in His Rasul (sallallahu ‘alaihi wasallam) and in whatever has been divinely revealed by Him.” Saying this, Hazrat ‘Umar (radhiyallahu ‘anhu) recited the shahaadah.
Rasulullah (sallallahu ‘alaihi wasallam) was overjoyed and called out the takbeer in a loud voice. Hearing the loud takbeer of Rasulullah (sallallahu ‘alaihi wasallam), all the Sahaabah (radhiyallahu ‘anhum) in the house realized that Hazrat ‘Umar (radhiyallahu ‘anhu) had embraced Islam.
(Extracted from Seeratul Mustafa 1/258-262)