Bukhari

Hadith Commentary:
Abu Dawood has narrated the complete hadith; in it, it is mentioned that Sahla said, "What do you command (should we observe veiling from Salim)?"
The Prophet (sallallahu alayhi wa sallam) replied, "Do so—give Salim your milk to drink."
She gave him milk to drink five times, and now he became like her foster son.
Aisha (radi Allahu anha) also, in accordance with this hadith, whenever she did not wish to observe veiling from someone, would instruct her nieces and the daughters of her brother to give him milk to drink, even if he was an adult.
But the wife of Umm Salamah (radi Allahu anha) and the other wives of the Prophet (sallallahu alayhi wa sallam) did not accept unveiling due to such (adult) nursing unless the nursing occurred in childhood.
They used to say, "Perhaps the Prophet (sallallahu alayhi wa sallam) granted this permission specifically for Salim; for others, such a ruling does not apply."
Qastallani said: This ruling was specific to Sahla and Salim, or it is abrogated; its discussion, in sha Allah, will come ahead.
The relevance to the chapter is as follows: Salim was a slave, yet Abu Hudhayfah married him to his niece, who was from the nobility of Quraysh.
Thus, it is known that in compatibility (kafa’ah), only consideration of religion is sufficient.
(Wahidi)

Hadith Commentary:
(1)
The complete hadith is as follows: Hazrat Sahla radi Allahu anha presented her case to the Messenger of Allah sallallahu alayhi wa sallam, saying:
"O Messenger of Allah! Now Salim has reached maturity, and it is very burdensome for Hazrat Abu Hudhayfa radi Allahu anhu that he comes and goes in the house. What do you command regarding him?" The Prophet sallallahu alayhi wa sallam said:
"You should give him milk to drink."
So she gave him milk to drink five times, and thus he became like her foster son.

(2)
The relevance of this hadith to the chapter heading is as follows: Hazrat Salim radi Allahu anhu was a freed slave of an Ansari woman, yet Hazrat Abu Hudhayfa radi Allahu anhu married him to his niece, who was from the nobility of Quraysh.
From this, it is understood that in being compatible (kafa’ah), only religion is sufficient; if, along with that, there is similarity in family and lineage, then that is better, but family and clan should not be given primary importance.
Those who, leaving aside religion, only consider tribalism will have to face many difficulties, trials, and hardships in the future. But if only religiosity is given consideration and family is given secondary importance, then, insha Allah, by the blessing of religion, this relationship will be successful.
And Allah knows best.

Hadith Commentary:
Imam Bukhari rahimahullah did not narrate the entire hadith.
In Abu Dawud, it is further mentioned that Sahla radi Allahu anha said, "O Messenger of Allah sallallahu alayhi wa sallam, we used to consider Salim radi Allahu anhu like a son. There was no veil (hijab) from him. Now, what do you command?" He sallallahu alayhi wa sallam said, "Do this: give Salim radi Allahu anhu milk to drink." She gave him milk to drink five times, and then Salim radi Allahu anhu was considered her foster son (through breastfeeding).
The practice of Aisha radi Allahu anha was upon this hadith.
The aforementioned Walid bin Utbah was killed at the Battle of Badr at the hands of Ali radi Allahu anhu.
Abu Hudhayfah radi Allahu anhu was his brother.
He had accepted Islam and was among the earliest emigrants (muhajirun).

Hadith Commentary:
➊
This hadith mentions that Abu Hudhayfah (radi Allahu anhu) participated in the Battle of Badr.

➋
His brother, Walid bin Utbah, was killed in this battle, but Abu Hudhayfah (radi Allahu anhu) accepted Islam and was among the earliest emigrants (muhajirun).

His respected wife, Sahla (radi Allahu anha), came to the Messenger of Allah (sallallahu alayhi wa sallam) and said:
Salim (radi Allahu anhu) has grown up, and because of this, some misgivings arise in the heart of Abu Hudhayfah (radi Allahu anhu) regarding his staying in the house. So the Messenger of Allah (sallallahu alayhi wa sallam) instructed her to breastfeed him. The complete details of this will be mentioned under the Book of Marriage, Hadith 5088.

Her full name is Sahla (radi Allahu anha) bint Suhail. She is not the same Sahla who freed Salim (radi Allahu anhu), because that Sahla belonged to the Ansar family, whereas this Sahla is from the Quraysh.

And Allah knows best.

In Islamic Shariah, a "mutabanna" (adopted, so-called son, or foster child) is neither considered a son nor an heir. He remains the son of his real father and is his heir. Similarly, attributing someone to a father other than his real one is also prohibited and unlawful, except in the case of attribution to forefathers, as when, during the Battle of Hunayn, the Messenger of Allah (sallallahu alayhi wa sallam) referred to himself as "Ibn Abd al-Muttalib." See: (Sahih al-Bukhari, al-Jihad wal-Siyar, Hadith: 2862, and Sahih Muslim, al-Jihad, Hadith: 1776), because he (Abd al-Muttalib) was more well-known and the Prophet’s (sallallahu alayhi wa sallam) father had passed away in his youth.