Muslim

� Lexical Explanation:
«بَــاءَ» Returned. That is, if the one who is called a disbeliever (kafir) is not actually a disbeliever, then the one who uttered it becomes a disbeliever.
The scholars interpret this statement of the Prophet sallallahu alayhi wa sallam as a warning and rebuke.
It also shows how grave a sin it is to utter such words; therefore, if someone says such a thing, it is obligatory for him to repent from it.

Hadith Commentary:
(1)
To call someone a disbeliever or to declare him a disbeliever is called "takfir."
The Ahl al-Hadith scholars have exercised great caution in the matter of takfir because this issue is extremely delicate.
It is a double-edged sword that is bound to strike one side.
If the person who is called a disbeliever is not, in reality, a disbeliever, then this disbelief will return upon the one who uttered it; that is, the one who said it will himself become a disbeliever. However, this will only occur if the person declares another a disbeliever without any kind of interpretation (ta'wil).
If he has a reasonable justification for calling the other person a disbeliever, then the one making the accusation will not himself become a disbeliever.
(2)
Imam al-Bukhari rahimahullah has restricted the application of the hadith with "reasonable interpretation" (ma‘qul ta'wil).
(3)
Our predecessors were extremely cautious in this regard.
They would not declare anyone who professed the kalimah (testimony of faith) to be a disbeliever.
They established certain rules and regulations regarding the issue of takfir, informed us of its causes and conditions, and also pointed out its preventatives.
As long as such conditions are not found in someone that would warrant declaring him a disbeliever, and there is no preventative present, one should absolutely refrain from declaring anyone a disbeliever.

Hadith Commentary: Benefits and Issues:
Declaring someone a disbeliever (kafir) on account of his evil deeds or bad character is a grave assertion.
A person should exercise control over his tongue,
and should not issue "fatwas of disbelief" indiscriminately.
However, there is no harm in saying that his actions are those of disbelief (kufr) or hypocrisy (nifaq), or that he is committing acts of polytheism (shirk),
because it is possible that due to ignorance, misunderstanding,
or even misinterpretation—whether incorrect or correct—or due to some interpretation,
and with sincerity and devotion to Allah, he may not consider that action to be an act of shirk or kufr.
But if a person denies the essentials of the religion, or declares something prohibited—established by definitive and certain proofs—to be permissible, and invites others to it as well,
then in such cases, the use of these words becomes permissible.
One who declares someone a disbeliever without justification,
himself comes under the threat of being declared a disbeliever, and ends up declaring himself a disbeliever.

Explanation:
1:
Because only a disbeliever (kafir) can call a believer (mumin) a disbeliever; a believer cannot call another believer a disbeliever. If he does so, and the one to whom it is said is truly deserving of being called a disbeliever, then he is indeed a disbeliever. Otherwise, the one who uttered it will, as a consequence of his statement, fall into the calamity of disbelief. Therefore, one should speak only after careful thought and should keep one's tongue under control.

Benefits and Issues:
The utterance of the tongue does not go in vain; no Muslim can be declared a disbeliever (kafir) without a clear and explicit Shar‘i (Islamic legal) proof.
If the person addressed is not actually deserving of it, then the one who uttered it is certainly affected by it.
However, this is of a lesser degree than major disbelief (kufr akbar).
It is a major sin (kabirah) for which the perpetrator has been declared a disbeliever in the same sense as mentioned in the above hadith.

Hadith Authentication: [واخرجه البخاري 6104، من حديث مالك، ومسلم 20، من حديث عبدالله بن ديناربه]
Jurisprudential Points
➊ It is forbidden and a major sin to declare a sound-believing Muslim brother as a disbeliever (takfir).
➋ One must completely avoid takfir and those who engage in takfir (takfiris). These people, instead of the disbelievers and polytheists, persist in declaring sound-believing Muslims as disbelievers.
➌ Declaring disassociation (bara’ah) from someone who is truly a disbeliever, polytheist, or deviant is a sign of faith, and this is the requirement of al-wala’ wal-bara’ (loyalty and disavowal).
➍ No one becomes a disbeliever by committing any sin other than shirk (polytheism) and kufr (disbelief), unless he considers it to be lawful (halal).
➎ The Messenger of Allah (sallallahu alayhi wa sallam) mentioned two men, one of whom was a reciter of the Qur’an and a defender of Islam. He declared his neighbor a polytheist (mushrik), issued a fatwa of shirk against him, and attacked him with a sword. The Messenger of Allah (sallallahu alayhi wa sallam) considered the one issuing the fatwa to be closer to shirk. See [صحيح ابن حبان الاحسان : 81 وسنده حسن وحسنه البزار فى البحر الزخار 7/220، 221 ح2793 والهيثمي فى مجمع الزوائد 1/187، 188]
➏ The famous Tabi‘i, ‘Ubaydah al-Salmani (rahimahullah), said: Every matter in which Allah is disobeyed is a major sin (kabirah). [شعب الايمان للبيهقي 1/273 ح293، وسنده صحيح]
➐ Sayyiduna ‘Abdullah ibn ‘Abbas (radi Allahu anhuma) said: Everything from which Allah has prohibited is a major sin (kabirah). [شعب الايمان للبيهقي : 7150 وسنده صحيح، نيز ديكهئے شعب الايمان : 292 وسنده حسن]