Bukhari

Hadith Commentary:
These were tears of happiness and joy.

Hadith Commentary:
➊
Ubayy ibn Ka'b radi Allahu anhu was a memorizer of the Noble Qur'an and an excellent reciter. On this basis, he became so accepted in the sight of the Lord of the worlds that Allah Himself commanded His beloved, Muhammad sallallahu alayhi wa sallam, to recite the Qur'an to him.
From this, one can estimate his good fortune.
➋
There was a possibility that Allah Ta'ala had given the Messenger of Allah sallallahu alayhi wa sallam the command to recite the Qur'an to some Companion from his Ummah, without specifying anyone by name.
Therefore, Ubayy radi Allahu anhu asked:
Did Allah Ta'ala mention my name? When he learned from the Messenger of Allah sallallahu alayhi wa sallam that Allah, the Lord of Might, had mentioned his name, he began to weep out of joy that the Master of the universe had honored this humble servant.
➌
It is also narrated from some scholars that Ubayy radi Allahu anhu wept out of fear that he might not be able to adequately give thanks for the extraordinary favors and blessings bestowed upon such an insignificant person.
According to our inclination, the first meaning is weightier and more likely.
And Allah knows best.

Hadith Commentary:

Due to being a memorizer (hafiz) and reciter (qari) of the Noble Qur’an, Ubayy ibn Ka‘b radi Allahu anhu attained such acceptance with Allah that Allah Himself commanded His beloved Messenger sallallahu alayhi wa sallam to recite the Noble Qur’an before Ubayy ibn Ka‘b radi Allahu anhu. Who can possibly estimate the fortune of such a person?

Hadith Commentary:
This surah is Madinan.
It contains eight verses.
He began to weep out of joy, thinking, "Where am I, a worthless servant, and where is He, the Sovereign of the heavens and the earth?"
Some have said that he wept out of fear, thinking, "How can I ever thank You for such grace and favor?"
The People of the Book and the polytheists of Arabia were so content with their false notions and corrupt beliefs that they would not abandon them at any cost. But Allah sent such an excellent Messenger, who was himself a living proof, that through his pure teachings, how many fortunate ones were guided to the straight path.
How many received guidance.
In Surah Bayyinah, Allah the Exalted has expressed this subject in the best manner, and has described the Noble Qur'an as ﴿صُحُفا مُطَهرَة﴾ ("purified scriptures") and the Noble Messenger with the word "Bayyinah" (clear proof).
Allah the Exalted has spoken the truth. We have believed in it and affirmed it. Our Lord, so write us among the witnesses. (Ameen)