Nasai

(1) In customary matters, permissibility remains established unless there is a prohibition. Since there is no prohibition regarding parting the hair, it is permissible to part the hair, and it is also permissible not to, because there is no command to do so. Both parting the hair and not parting it are established from you (the Prophet sallallahu alayhi wa sallam). The fundamental reason for this is that the Shariah has not given any specific ruling regarding it. Depending on circumstances, either option can be chosen. In such matters, your (the Prophet’s) agreement with the People of the Book was for the purpose of winning their hearts, so that perhaps they might incline towards Islam. However, when you perceived that even agreement with them was not beneficial, you abandoned this agreement. The Messenger of Allah (sallallahu alayhi wa sallam) also preferred agreement with the People of the Book because, at least in claim, they professed to be followers of a revealed religion. In contrast, the polytheists were staunch idolaters.

(2) The hair parting should be done in the middle of the head, because the blessed habit of the Messenger of Allah (sallallahu alayhi wa sallam) was to part the hair in the middle. And Allah knows best.

Hadith Footnote:
And he stopped hanging it on his forehead.
Perhaps you have received the command.

Hadith Commentary:
1.
The Messenger of Allah (sallallahu alayhi wa sallam) preferred to conform with the People of the Book because, at the very least, they claimed to be adherents of a revealed religion, and it was also intended that perhaps, through this conduct, they might come closer to Islam. However, when you (sallallahu alayhi wa sallam) became disappointed with their acceptance of Islam and misfortune prevailed over them, you were commanded to oppose them in many matters.
Fasting on the Day of Ashura, facing the Qiblah, social interaction with menstruating women, and the matter of the Sabbath were all based on this reasoning.
Among the polytheists, idol worship was widespread, so you (sallallahu alayhi wa sallam) would oppose them, but this opposition was in those matters where the Sunnah of Ibrahim (alayhis salam) was not evident.
When you (sallallahu alayhi wa sallam) learned that a particular practice was from the Sunnah of Ibrahim (alayhis salam), you did not care about conforming with the polytheists. The examples of this are clear in the rites of Hajj.

2.
It is narrated from Anas (radi Allahu anhu) that the Messenger of Allah (sallallahu alayhi wa sallam) used to comb his hair and let it fall over his forehead, but later he would part his hair.
(Musnad Ahmad: 215/3)
When the Messenger of Allah (sallallahu alayhi wa sallam) applied perfume before entering the state of ihram, its fragrance would remain in his parting for a long time, as Aisha (radi Allahu anha) states that she could still see the shine of the perfume in the parting of the Messenger of Allah (sallallahu alayhi wa sallam) which had been applied at the time of ihram.
(Sahih al-Bukhari, Book of Hajj, Hadith: 1538)

Hadith Commentary:
Perhaps later on, the command regarding this was revealed to you.
You abandoned letting the hair hang over the forehead; now this has remained the practice of the Christians.
It is obligatory upon Muslims that they adopt only the ways, manners, and conduct of their Noble Messenger (sallallahu alayhi wa sallam), and under no circumstances should they adopt the erroneous customs of others.

Hadith Commentary:
1.
Since the People of the Book adhered to a divinely revealed law, whenever the Messenger of Allah (sallallahu alayhi wa sallam) did not receive any command from Allah regarding certain matters, he would prefer to conform with them. Then, when it was revealed to him through revelation that parting the hair is from the religion of Ibrahim, he acted upon this. After that, he no longer concerned himself with conforming to the practices of the polytheists.
2.
This hadith mentions a custom of the Jews, the opposition to which has been emphasized. For this reason, Imam Bukhari (rahimahullah) has mentioned this hadith here.

Hadith Commentary:

Placing the hair of the head in the Sunnah manner is better in every respect, but nowadays, the epidemic of fashion has spread—especially the trend of hippie-ism, where keeping the hair in a certain way distorts one’s appearance. This fashion is an extreme sin, a distortion of Allah’s creation, and an imitation of the disbelievers.

The youth of Islam are in dire need of striving (jihad) against such wrongful practices.

Such fashion is considered objectionable even in the eyes of non-Muslims; therefore, Muslims should never adopt it.

Hadith Commentary:
The parting of the hair by the Messenger of Allah (sallallahu alayhi wa sallam) was by the command of Allah, the Exalted.
Although the polytheists also used to part their hair, the Prophet (sallallahu alayhi wa sallam) did not do so in imitation or following of them. Therefore, only that resemblance to the disbelievers and polytheists is impermissible which is their religious symbol or a distinct national mark. However, making a crooked (side) parting is contrary to the Sunnah of the Messenger of Allah (sallallahu alayhi wa sallam); rather, it is among the negative influences of Western culture. Therefore, Muslims should refrain from this bad habit.

Hadith Commentary:
Vocabulary of the Hadith:
(1)
yasdiloon:
Letting the hair hang down over the forehead,
leaving it loose.
(2)
yufarriqoon:
Parting the hair.

Benefits and Issues:
In the beginning, the Messenger of Allah (sallallahu alayhi wa sallam), in matters where there was no explicit command, would conform to the People of the Book in order to make them feel at ease. However, when they persisted in stubbornness and obstinacy, and most of the polytheists had become Muslim, he began to oppose the practices of the People of the Book.

Benefits and Issues:
It is clear that the extracting of the forelock by the Messenger of Allah (sallallahu alayhi wa sallam) was by the command of Allah, even though the polytheists also used to extract it.
Thus, resemblance to the polytheists and disbelievers is only impermissible in those matters which are their religious or specifically national symbols.

Commentary:
Benefits and Issues:

➊
In Makkah Mukarramah, the majority were polytheists.
The Messenger of Allah (sallallahu alayhi wa sallam) would adopt the style of the People of the Book to distinguish himself from them.
When the Messenger (sallallahu alayhi wa sallam) migrated, he adopted a different style in order to differentiate from the many People of the Book present in Madinah.

➋
Every action of the Messenger of Allah (sallallahu alayhi wa sallam) was in the light of revelation; therefore, leaving the hair loose without parting it is abrogated, and parting the hair is Sunnah and meritorious.