It is narrated from Sayyiduna Abu Dharr (may Allah be pleased with him), he says: I heard the Prophet (peace and blessings of Allah be upon him) say: "When a man calls another man a disbeliever or a sinner, and he is not so, then this (statement) returns upon him (and he becomes as such)."
Hadith Referenceالادب المفرد / كتاب السباب / 432
Hadith Gradingالألبانی:صحيح
Hadith Takhrij«صحيح : أخرجه البخاري ، كتاب الأدب : 6044 و مسلم : 61 - انظر الصحيحة : 2891»
Explanation & Benefits
Shaykh Maulana Usman Muneeb
Benefits and Issues: There is no harm in calling a disbeliever (kafir) a disbeliever; however, one should avoid calling any Muslim a disbeliever (kafir) or a transgressor (fasiq), because there is a risk that the person himself may become such. However, if someone truly is as such, then that is another matter. Similarly, if someone does this to belittle or reproach another, he will also be sinful. Therefore, one should not be hasty in issuing such a verdict (fatwa) against a Muslim.
Source: Fadlullah al-Ahad: Urdu Commentary on al-Adab al-Mufrad, Page: 432