Ibn Umar, may Allah be pleased with him, narrates that the Messenger of Allah, peace and blessings be upon him, said: "If a person says to his (Muslim) brother: 'Disbeliever,' then surely one of them will return to disbelief (from faith)." Agreed upon.
Hadith Referenceمشكوة المصابيح / كتاب الآداب / 4815
Hadith Gradingالألبانی:مُتَّفَقٌ عَلَيْهِ | زبیر علی زئی:متفق عليه
Hadith Takhrij´تحقيق و تخريج: محدث العصر حافظ زبير على زئي رحمه الله` «متفق عليه، رواه البخاري (6104) و مسلم (111/ 60)»