Sayyiduna Abdullah bin Umar (may Allah be pleased with them) narrates that the Messenger of Allah (peace and blessings be upon him) said: Whoever calls his brother a disbeliever (kafir), then one of the two becomes a disbeliever.
Hadith Referenceاللؤلؤ والمرجان / كتاب الايمان / 39
Hadith Gradingمحدثین:«صحیح»
Hadith Takhrij«صحیح، أخرجه البخاري في: 78 كتاب الأدب: 73 باب من كفر أخاه بغير تأويل»