It is narrated from Sayyiduna Abdullah bin Mas'ud (may Allah be pleased with him), he said: There is a veil from Allah between every two Muslims. When one of them speaks abusively, he tears the veil of Allah. And when one of them says to the other, "You are a disbeliever," then one of them becomes a disbeliever.
Hadith Referenceالادب المفرد / كتاب السباب / 435
Hadith Gradingالألبانی:ضعيف
Hadith Takhrij«ضعيف : أخرجه الطبراني فى الكبير : 224/10 و البيهقي فى شعب الإيمان : 5017»
Explanation & Benefits
Shaykh Maulana Usman Muneeb
Benefits and Issues: The point is that if the one who says it is truthful, then it is correct; otherwise, he himself becomes a disbeliever (kafir). For the authentic hadith with this meaning and its benefits, see Hadith number: 432.
Source: Fadlullah al-Ahad: Urdu Commentary on al-Adab al-Mufrad, Page: 435