Bukhari

Hadith Commentary:
Salim (radi Allahu anhu) was originally Persian and was a slave of the wife of Hudhayfah (radi Allahu anhu). He was highly learned and a reciter (qari) of the Qur'an.

Hadith Commentary:
➊
The Messenger of Allah (sallallahu alayhi wa sallam) specified these four noble Companions (radi Allahu anhum) because they possessed great expertise in preserving the words of the Noble Qur’an, in their recitation, and in understanding their meanings, even though other Companions (radi Allahu anhum) had greater insight into the meanings of the Qur’an than them. These individuals used to recite the Qur’an directly to the Messenger of Allah (sallallahu alayhi wa sallam). Among these Companions was also Hazrat Salim (radi Allahu anhu), who was highly learned and a reciter (qari) of the Qur’an. (Fath al-Bari: 7/129)

➋
It should be noted that the number four mentioned in the hadith is not for restriction, but rather indicates their virtue.

Hadith Commentary:
In the blessed era of the Prophet (sallallahu alayhi wa sallam), these individuals were regarded as distinguished experts of the Noble Qur’an. Therefore, the Prophet (sallallahu alayhi wa sallam) appointed them as teachers of the Noble Qur’an. Only the people of faith can truly comprehend how great an honor this is.

Hadith Commentary:
Hazrat Mu'adh bin Jabal radi Allahu anhu belonged to the tribe of Khazraj.
His kunyah was Abu Abdur-Rahman.
The Messenger of Allah sallallahu alayhi wa sallam appointed him as the governor and sent him to Yemen.
He returned to Madinah from there.
Then he went to the region of Sham (Greater Syria).
He spent his life in jihad, and passed away during the plague of ‘Amwas in Rabi‘ al-Awwal, 18 AH, corresponding to March 639 CE.
The Messenger of Allah sallallahu alayhi wa sallam praised him and also said:
“The one who knows the most about what is lawful and unlawful is Mu'adh bin Jabal radi Allahu anhu.”
(Jami‘ at-Tirmidhi, al-Manaqib, Hadith: 3790, 3791)
Hazrat ‘Umar radi Allahu anhu said regarding him:
Whoever wishes to gain understanding of religious matters should sit as a student before Hazrat Mu'adh bin Jabal radi Allahu anhu.
(Fath al-Bari: 7/159)

Hadith Commentary:

Among them, Abdullah ibn Mas'ud and Salim are from the Muhajirun, while Mu'adh and Ubayy ibn Ka'b are from the Ansar. These companions were the foremost scholars and memorizers of the Noble Qur'an. Although there were other companions who were reciters of the Qur'an, these four had memorized the Qur'an the most.

Hadith Commentary:
1.
Among these individuals, Abdullah ibn Mas'ud radi Allahu anhu and Salim, the freed slave of Hudhayfah radi Allahu anhu, were from the Muhajirun, and Mu'adh ibn Jabal radi Allahu anhu and Ubayy ibn Ka'b radi Allahu anhu were from the Ansar.
These individuals were experts in the Noble Qur'an and had a special devotion to its memorization and recitation.
Although other noble Companions radi Allahu anhum ajma'in were also memorizers of the Qur'an, these four had memorized the most of the Noble Qur'an.
Allamah Kirmani rahimahullah has written that the mention of these four indicates their remaining for a longer period, but this point is questionable, because Salim ibn Ma'qil radi Allahu anhu was martyred in the Battle of Yamamah.
Mu'adh ibn Jabal radi Allahu anhu passed away during the caliphate of Umar radi Allahu anhu, and besides him, Abdullah ibn Mas'ud radi Allahu anhu and Ubayy ibn Ka'b radi Allahu anhu passed away during the caliphate of Uthman.
However, Zayd ibn Thabit radi Allahu anhu remained alive for a long time after them, and he alone became the reference point for the general public and the elite in matters of Qur'anic recitation and sciences.
(Fath al-Bari: 9/60)

Hadith Commentary:
1.
At the time of his death, Mu'adh ibn Jabal (radi Allahu anhu) advised that knowledge should be acquired from four individuals:
Abu Darda (radi Allahu anhu), Salman al-Farsi (radi Allahu anhu), Abdullah ibn Salam (radi Allahu anhu), and that one should become a student of Abdullah ibn Mas'ud (radi Allahu anhu).
(Fath al-Bari: 7/129)
2.
In any case, Abdullah ibn Mas'ud (radi Allahu anhu) saw the Messenger of Allah (sallallahu alayhi wa sallam) from very close proximity, and for this reason, he possesses great eminence.
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radi Allahu anhu.
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Hadith Commentary:
➊
Ubayy ibn Ka'b radi Allahu anhu belonged to the tribe of Najjar.
He was present at the Second Pledge of Aqabah.
He participated in the Battle of Badr and other battles.

➋
According to the mentioned hadith, the Messenger of Allah sallallahu alayhi wa sallam recited the Qur'an to Ubayy ibn Ka'b radi Allahu anhu by the command of Allah Almighty.
This is such a virtue of his in which no other companion shares.
The Messenger of Allah sallallahu alayhi wa sallam recited the Qur'an to him in order to manifest his virtue, so that people would be encouraged to learn the Qur'an from him, and indeed, this is what happened.
After the Messenger of Allah sallallahu alayhi wa sallam, he became a renowned imam in the recitation of the Qur'an, and in fact, he became famous with the title "Sayyid al-Qurra" (Leader of the Reciters).
('Umdat al-Qari: 1/520)

Hadith Commentary:
Benefits and Issues:
➊ These four individuals had devoted themselves to the Qur'an.
➋ During the Prophet’s (sallallahu alayhi wa sallam) lifetime, they learned as much as possible from him,
➌ memorized it in his style and manner, and after him, dedicated themselves to teaching the Qur'an to others and continued to teach it after his passing.
➍ Therefore, the Prophet (sallallahu alayhi wa sallam) encouraged learning the Qur'an from them.

Explanation:
1:
The meaning is that these four Companions were especially distinguished in learning the Qur’an directly from the Prophet (sallallahu alayhi wa sallam), memorizing it, and reciting it in the best manner. This does not mean at all that other Companions did not memorize the Qur’an, nor does it mean that it is necessary to strictly adhere to the recitations narrated from them. This is because Uthman (radi Allahu anhu), for the highest purpose, had all recitations except one abolished (by “recitations” here, what is meant is not the well-known seven modes of recitation, but rather the personal codices of the Companions).