The Condemnation of Unauthorized Takfir by the Prophet Himself صلى الله عليه وسلم
The rejection of unjustified takfir against a Muslim is explicitly narrated from the Prophet صلى الله عليه وسلم himself. Accordingly, this act is described as a grave sin.
Thus, al-Bukhari reports in his Sahih collection the following hadith from Abu Hurayrah رضي الله عنه, from the Prophet صلى الله عليه وسلم:
عَنْ أَبِي هُرَيْرَةَ رَضِيَ اللَّهُ عَنْهُ أَنَّ رَسُولَ اللَّهِ ﷺ قَالَ «إِذَا قَالَ الرَّجُلُ لِأَخِيهِ يَا كَافِرُ فَقَدْ بَاءَ بِهِ أَحَدُهُمَا»
If a person says to his brother in faith: “You kafir!”, then one of the two returns with it upon himself.
Al-Bukhari narrates this hadith in his Sahih collection in various narrations. Likewise, the hadith is also found in Sahih Muslim. In the different narrations, it sometimes states, “then one of them has returned with it (bihi)”, and in another wording, “has returned with her (biha)”, which led scholars to the question of what exactly the person “returns with” in this case. It is undisputed that the illegitimate accusation of a Muslim being a non-Muslim constitutes a major sin.
Clearly, this applies when takfir is carried out without a legitimate justification in the shari'ah. However, if a person were to, for example, profess Islam but then begin to worship cows, it would be an entirely different case.
In such a situation, the matter is so clear that not a single Muslim would be allowed to consider this person as a brother in faith. If one were to designate a person as a Muslim despite his evident polytheism, this would be an indication that one has not properly understood the core message of Islam itself. Otherwise, how could someone declare a person to be a Muslim despite idol worship or similar practices.
However, in reality, some Muslims practice takfir merely by following their own inclinations. The point here is to highlight precisely this issue.
In another narration of the aforementioned hadith, which al-Bukhari mentions in his book al-Adabu l-Mufrad, it states:
عَنِ ابْنِ عُمَرَ، عَنِ النَّبِيِّ ﷺ قَالَ «مَنْ كَفَّرَ أَخَاهُ، فَقَدْ بَاءَ بِهَا أَحَدُهُمَا»
Whoever accuses his brother in faith of kufr, then one of them returns with it.
Here, the verb “kaffara” is explicitly used, which all the more clearly demonstrates the connection of these narrations to the topic being discussed here.