Ibn Majah

Hadith Commentary:
It is understood from this that the unlawful killing of a Muslim brings a person close to disbelief (kufr).
Or, it may refer to that killing which is committed considering it lawful; in that case, the person would indeed become a disbeliever (kafir).

Hadith Commentary:
➊
We have determined eight meanings of this hadith under Hadith 6869; they must certainly be consulted.
In any case, the unlawful shedding of a Muslim’s blood is a very grave sin, which the Messenger of Allah (sallallahu alayhi wa sallam) has described as kufr, and by kufr is meant the concealing of the truth—that is, after me, do not conceal the truth such that you begin to strike each other’s necks, because a Muslim is supposed to defend another Muslim.
This is his right, and if he fights against him, it is as if he is concealing his right.

➋
Another meaning is that the mentioned act leads a person towards kufr, because whoever becomes accustomed to committing major sins, there is then a danger that he will fall into an even greater crime—that is, his end may not be upon Islam.
In any case, in times of tribulation, a person must act with utmost caution.
(Fath al-Bari: 13/35)

Hadith Commentary:
Vocabulary of the Hadith:
:
وَيْلٌ:
Used for expressing amazement and grief.
And وَيْحٌ is used on occasions of mercy and compassion.

Benefits and Issues:
(1)
For Muslims to fight and wage war against each other, and to consider it permissible and correct,
is tantamount to declaring each other disbelievers,
and its evil consequences can reach as far as disbelief (kufr).

(2)
Mutual fighting among Muslims is a violation of each other's rights,
because there is a bond of brotherhood and chivalry among them which necessitates mutual cooperation and support.

(3)
A Muslim does not fight Muslims,
rather, the fighting of Muslims is against belligerent disbelievers (harbi kuffar),
therefore, mutual fighting is to adopt the ways and conduct of disbelievers, and Muslims have been prohibited from resembling them and adopting their behavior.

Benefits and Issues:
Evil deeds cause a decrease in faith (iman); for Muslims to kill one another is among the worst of deeds. This is evidence of a deficiency in faith, which has been described as disbelief (kufr), but due to this, a person does not leave the fold of the religion (millet).

However, in the case of accidental killing (qatl khata), the killer’s blood relatives, in fact the entire clan, will join with him in paying the blood money (diyah). This is not contrary to this narration, because accidental killing (khata) is not a crime, and paying the blood money for the slain is not a punishment for the relatives; rather, it is simply cooperation with the person from whom the killing occurred unintentionally and without purpose. And the blood of a Muslim who has been killed cannot be left unavenged. However, if the killer committed murder deliberately (qatl amd), then retribution (qisas) will be taken from him alone, and if the matter is settled upon blood money (diyah), then he alone will pay it. No responsibility will be placed upon the relatives, because he is a criminal, and what cooperation is there with a criminal?

(1) Fighting against Muslims is the work of disbelievers. If Muslims begin to fight one another, they become similar to the disbelievers, and furthermore, the objective of the disbelievers is fulfilled; they themselves no longer need to fight. Whoever considers fighting permissible on the basis of mutual differences is, in reality, a disbeliever, because he declares something forbidden to be lawful. However, if someone fights merely out of emotion, then he will not be a disbeliever, but his action will resemble that of the disbelievers. In such a case, if he kills someone, he will be killed in retribution (qisas).

(2) Sometimes, due to misunderstanding, war breaks out, or mischief-makers incite both parties to fight; in such cases, neither party becomes disbelievers, as happened in the battles of Jamal and Siffin. Aisha, Zubair, Talha, Muawiyah, and Amr ibn al-As (radi Allahu anhum) were seeking retribution for the unjust killing of Uthman (radi Allahu ta'ala anhu), but the killers of Uthman, in order to save their own necks, instigated war. Ali (radi Allahu ta'ala anhu) considered this manner of demanding retribution as rebellion and regarded suppressing the rebellion as a governmental duty, but the matter was not so simple. The conspiracies of non-Muslims were quite deep. Such misunderstandings had arisen between the parties that, even unwillingly, fighting continued between them, even though both parties were well-intentioned. Their being Companions (Sahabah) alone is sufficient proof of their good intentions. The Companions were not ordinary people; rather, ﴿أُولَئِكَ الَّذِينَ امْتَحَنَ اللَّهُ قُلُوبَهُمْ لِلتَّقْوَى﴾ (al-Hujurat: 39:3) — they were those whom Allah had tested their hearts for piety. They were the chosen individuals of Allah, so it is necessary to have the best possible opinion about them; otherwise, one's own faith is at risk. Those people are certainly destined for Paradise. There are explicit glad tidings from the Messenger of Allah (sallallahu alayhi wa sallam) for them by name. Whoever harbors ill thoughts about them is deprived of faith. Radi Allahu anhum wa ardaahum.

(3) "Do not become disbelievers" — one meaning of "disbeliever" is also "ungrateful." Fighting among yourselves is ingratitude for the blessing of faith.

(1) The clarification of Imam Nasa’i rahimahullah’s statement is as follows: The aforementioned narration has been reported as connected (muttasil) by some narrators and as mursal by others. Imam Nasa’i rahimahullah states that the narration being connected is not correct; rather, the correct view is that this narration is mursal. This is because the narrators who report it as connected are Sharik and Abu Bakr ibn Ayyash, and both of them narrate from A’mash. Besides Abu Bakr ibn Ayyash and Sharik, this narration has also been reported from A’mash by Abu Mu’awiyah and Ya’la. Firstly, Sharik is a narrator who makes many mistakes (kathir al-khata’), and secondly, both he and Abu Bakr ibn Ayyash have contradicted Abu Mu’awiyah, even though Abu Mu’awiyah is the most reliable (athbat) of all A’mash’s students, except for Sufyan al-Thawri. Abu Mu’awiyah has reported this narration as mursal. Furthermore, Ya’la ibn ‘Ubayd has also followed Abu Mu’awiyah in reporting it as mursal.

(2) “Do not become disbelievers (kuffar)”—it has also been interpreted to mean: Do not become apostates (murtadd) and disbelievers after me, otherwise your condition will revert to what it was before Islam, when you used to strike each other’s necks and there was widespread killing and fighting among you. And Allah knows best.

(3) “Will not be seized”—this is the golden principle of Islam: every person is responsible for his own actions. For someone’s crime, his brother, father, or son cannot be seized, unless their involvement in the crime is proven. In the days of ignorance (jahiliyyah), it was a common practice that instead of the killer, it was considered permissible to kill any relative of his, even any member of his tribe. Because of one person’s crime, his entire tribe would be considered guilty, and thus killing and fighting were widespread. Sometimes, one murder would result in hundreds of killings. Islam has negated and condemned this lack of principle.