Bukhari

Hadith Commentary:
(1)
Apparently, from this hadith it is understood that whoever swears an oath by any religion other than Islam, for example:
“If I do this deed, then I will become a Jew or a Christian,” the Islam of the one who says such a thing is put in danger.
It is possible that he may actually become a Jew, just as he has said.
(2)
The meaning of the statement of the Messenger of Allah (sallallahu alayhi wa sallam) appears to be a warning and threat, and a promise of punishment, as if he becomes deserving of the punishment of the Jews.
In another hadith, this is further clarified. Hazrat Buraydah (radi Allahu anhu) says that the Messenger of Allah (sallallahu alayhi wa sallam) said:
“Whoever swears an oath and says, ‘I am free from Islam,’ if he is lying, then he will indeed be free from Islam as he has said, and if he is truthful, then even so, he will not be able to return to Islam in a sound state.”
(Sunan Abi Dawud, Al-Ayman wa al-Nudhur, Hadith: 3258)
In a narration of Sahih al-Bukhari it is stated:
“Whoever deliberately swears a false oath by any religion other than Islam, he is as he has said.”
(Sahih al-Bukhari, Al-Jana’iz, Hadith: 1363)
(3)
Hafiz Ibn Hajar (rahimahullah) writes that suspending something upon a condition is also considered an oath, because it also involves a commitment to do or not do something, and this is the very purpose of an oath.
The majority (jumhur) hold the view that such a person should repent and seek forgiveness, and there is no expiation (kaffarah) due upon him. However, there is some detail in this: if he intends to honor or venerate any religion other than Islam, then without doubt he will be excluded from the religion of Islam; and if veneration is not intended, but only suspension (ta‘liq) is intended, then if he intends to adopt another religion, he becomes a disbeliever (kafir), because the intention of disbelief is itself disbelief; and if he wishes to remain distant from it, then he will not become a disbeliever.
(4)
In any case, the inclination of Imam Bukhari (rahimahullah) appears to be that such a person should not be considered outside the fold of the religion.
(Fath al-Bari: 11/656)

Hadith Commentary:
To swear an oath upon any religion, for example, to say: "If I do this act, then I will become a Jew or a Christian, etc.," is a very evil oath.
May Allah protect us from it.

Hadith Commentary:
(1)
An oath upon any religion or community other than Islam is such that a person says:
If I do this, then I am a Jew or a Christian.
If he is lying in this oath, then he will become a Jew or a Christian, because doing so is to venerate Judaism or Christianity, and venerating any religion other than Islam is disbelief (kufr).

(2)
The second sentence of this hadith, that to curse (la'nat) a believer is like killing him, is because the meaning of la'nat is to distance someone from the mercy of Allah, and killing someone also results in distancing him from worldly life.

(3)
The final sentence is that to attribute a Muslim towards disbelief (kufr) is like killing him.
The reason for this analogy is that disbelief (kufr) leads to killing, so the one who attributes someone to disbelief has, in effect, attributed him to the cause of being killed, as if he has killed him.
The purpose of Imam Bukhari rahimahullah is that if the attribution of disbelief towards someone is due to a reasonable interpretation (ma‘qool ta’wil), then it will not be like killing, i.e., he will not be deserving of the warning mentioned in the hadith.
And Allah knows best.

Hadith Commentary:

Hazrat Thabit bin Dhahak radi Allahu anhu was among those noble companions who pledged allegiance (bay‘ah) for jihad at the blessed hand of the Messenger of Allah sallallahu alayhi wa sallam beneath a tree on the occasion of the Treaty of Hudaybiyyah. This is mentioned in Surah al-Fath, where Allah states that He was pleased with those believers who, willingly and wholeheartedly, pledged allegiance for jihad at the blessed hand of the Prophet sallallahu alayhi wa sallam beneath the tree. The content of the hadith is clear.

Hadith Commentary:
(1)
To swear by any religion other than Islam means that if I do such-and-such act, then I am a Jew or a Christian; in such a case, he will become exactly what he has said.

(2)
In this hadith, five rulings have been mentioned, the explanation of which we will provide later; for now, considering the heading, it is stated that to curse a believer is a very grave crime, as if one has killed him, because the meaning of "la'nat" (curse) is to distance someone from the mercy of Allah. In this way, by cursing him, one wishes to deprive him of the benefits of the Hereafter.

It is mentioned in the hadith:
“When a servant curses someone, the curse rises towards the sky, but the gates of the sky are closed to it. Then it descends towards the earth, but the gates of the earth are also closed to it. Then it turns to the right and to the left; if it does not find any place, it falls upon the one who was cursed, provided he is deserving of it; otherwise, the curse returns to the one who uttered it.”
(Sunan Abi Dawud, Al-Adab, Hadith: 4905)

Hadith Commentary:
Benefits and Issues:
(1)
Swearing an oath by any religion other than Islam means that if I have done such-and-such act, then I am a Jew or a Christian, even though he has already done that act. This means that he has abandoned Islam for his own benefit and has sacrificed Islam for worldly gain. In the same way, he has preferred the world over the Hereafter. If he has done this act knowingly, then he will truly be upon something other than Islam. And if he has done this in order to create emphasis to conceal his lie or wrongful act, then even so, this act is so grave that it is as if he has left the religion, because he has considered a false oath to be insignificant and has acted in a manner contrary to the reverence and veneration of Allah, which can be a cause of disbelief (kufr).
(2)
A vow (nadhar) should be made only regarding something that is within a person's ability or ownership; otherwise, such a vow will be vain and useless.

Explanation:
1:
This is for the purpose of severity and warning; if one holds the correct creed, he will not be considered a disbeliever (kafir).

(1) The form of this situation is as follows: if a person says, “If I have done such-and-such act, then I become a Jew or a Christian, etc.,” while in reality he has done that act and he also remembers it. Or, “If I do this act, then I become a Jew or a Christian,” while his intention is indeed to do the act, and he swears only for the sake of deception. It is evident that this person has chosen to be a Jew or a Christian. It is as if he is indeed a Jew or a Christian.

(2) “He will continue to be punished,” meaning from his death until the Resurrection. After that, based on the totality of his deeds, a decision will be made regarding his entry into Paradise or Hell. This is his fate.

The human soul is not his own possession; rather, it is the possession of Allah Ta'ala. It is not permissible to dispose of it in a manner contrary to the will of Allah Ta'ala, such as killing oneself or keeping oneself hungry and thirsty, etc.

For details, see Hadith number 3801.

Commentary:
Benefits and Issues:

➊
The meaning of swearing an oath by another religion is that a person says:
“If I have done such-and-such, then I am a Jew,” or says:
“If I lie, I will become a disbeliever (kafir).”
One should avoid taking oaths in this manner.

➋
Hafiz Salahuddin Yusuf (hafizahullah) writes regarding this that if, at the time of swearing the oath, his intention was indeed that if he did this act, he would actually adopt the path of disbelief (kufr), then he will immediately become a disbeliever (kafir). However, if his purpose was to express steadfastness upon the religion of Islam, and his resolve was that he would never adopt the path of disbelief (kufr), then he will not become a disbeliever (kafir), but the method he adopted was incorrect. Therefore, he should engage in repentance (tawbah) and seeking forgiveness (istighfar), and it is even better that he recite the testimony of faith (kalimah shahadah) again and renew his Islam. See: (Riyadh as-Salihin (Urdu), Volume Two, Benefits of Hadith: 1710, published by Darussalam)