Bukhari

Hadith Commentary:
Its story is mentioned in full in the commentaries.
It is said that the Messenger of Allah (sallallahu alayhi wa sallam), with the condition that if Zayd divorced Zaynab of his own free will and Zaynab was also pleased, then he (sallallahu alayhi wa sallam) would include her among his wives, kept this matter concealed in his heart due to it being against the customary practice of the people.
This is what is alluded to in the verse.
The statement of Aisha (radi Allahu anha) is absolutely correct that if the Messenger of Allah (sallallahu alayhi wa sallam) had wished to conceal any verse of the Qur'an, he would have concealed this verse, but as soon as it was revealed to him, he conveyed it in full to the Ummah (sallallahu alayhi wa sallam).
Later, by marrying Zaynab (radi Allahu anha), he broke an incorrect custom of the Age of Ignorance (Jahiliyyah).
In the Age of Ignorance, an adopted son was considered a real son, and marriage with his wife was considered unlawful.
He (sallallahu alayhi wa sallam) abolished both of these customs.

Hadith Commentary:
➊
The familial status of Lady Zaynab radi Allahu anha was very high, whereas Zayd ibn Harithah radi Allahu anhu, outwardly, had borne the stigma of slavery before being set free. Therefore, there could not be a compatibility of temperament, nor could the social disparity be so quickly erased from minds. Lady Zaynab radi Allahu anha was not able to show her husband, Zayd ibn Harithah radi Allahu anhu, the respect that she ought to have shown. When discord began between them, Sayyiduna Zayd radi Allahu anhu would complain about this to the Messenger of Allah sallallahu alayhi wa sallam and say that he wished to divorce her and set her free. However, the Messenger of Allah sallallahu alayhi wa sallam would always advise him that since this woman had sacrificed her own desire and married you, you too should exercise some patience.
Now, if you divorce her, she and her relatives will consider it a disgrace. Fear Allah and do not let minor issues lead to discord.
But when the complaints became frequent, it is possible that the thought occurred in your sallallahu alayhi wa sallam’s heart that if Zayd divorced Zaynab, then the only way to console Zaynab would be to marry her.
However, there was also the fear of the slander of the hypocrites, that he had taken the wife of his adopted son into his own household. This was the matter that the Prophet sallallahu alayhi wa sallam kept concealed in his heart, as Lady Aishah radi Allahu anha states: If Muhammad sallallahu alayhi wa sallam were to conceal anything from the Qur’an, he would surely have concealed this verse.
(Sahih Muslim, Al-Iman, Hadith: 440. (177))

➋
Although Allah Ta’ala does not hold people accountable for thoughts in their hearts, in view of the exalted status of the Prophet sallallahu alayhi wa sallam, even such a thought arising in his heart was not befitting his rank. Therefore, even for such a thought, a warning was given to the Prophet sallallahu alayhi wa sallam from Allah Ta’ala.
During this period, Allah Ta’ala informed the Messenger of Allah sallallahu alayhi wa sallam that He would give Lady Zaynab radi Allahu anha to him in marriage, so that the evil custom of the Age of Ignorance could be abolished—that marrying the wife of an adopted son is forbidden. Thus, when Zayd radi Allahu anhu divorced her and her waiting period (‘iddah) had passed, without any formal proposal or acceptance, Allah Ta’ala Himself married Lady Zaynab radi Allahu anha to the Messenger of Allah sallallahu alayhi wa sallam.
This is why Lady Zaynab radi Allahu anha used to say, as a mark of honor, that while the other wives’ marriages were arranged by their guardians, her marriage was performed by Allah Ta’ala Himself above the seven heavens.
(Sahih al-Bukhari, Al-Tawhid, Hadith: 7420)

➌
Hazrat Anas radi Allahu anhu narrates that when Lady Zaynab radi Allahu anha’s waiting period (‘iddah) was completed, the Messenger of Allah sallallahu alayhi wa sallam said to Zayd radi Allahu anhu: Go and mention me to Zaynab.
Zayd radi Allahu anhu says: In compliance with the order, when I went to her, she was preparing the dough for bread. Because of her exalted status, I could not look at her directly, since the Messenger of Allah sallallahu alayhi wa sallam had mentioned her.
Then I said to her:
Zaynab radi Allahu anha! The Messenger of Allah sallallahu alayhi wa sallam has sent me; he remembers you. She replied: I will not say anything until I have consulted my Lord (performed istikhara). Then she stood on her prayer mat.
Meanwhile, the Qur’an was revealed and the Messenger of Allah sallallahu alayhi wa sallam went to her without any further command.
(Sahih Muslim, Al-Nikah, Hadith: 3502(1428))
In any case, this was the command of Allah Ta’ala: that an adopted son (mutabanna) does not have the status of a real son, such that his divorced wife would become forbidden, and Allah Ta’ala wanted to abolish this erroneous custom, and for this, the Messenger of Allah sallallahu alayhi wa sallam himself was chosen.