It is narrated from Sayyiduna Abdullah bin Umar (may Allah be pleased with them both) that the Messenger of Allah (peace and blessings be upon him) said: "Whoever calls his Muslim brother a disbeliever, then surely one of them returns with it."
Hadith Referenceالادب المفرد / كتاب السباب / 439
Hadith Gradingالألبانی:صحيح
Hadith Takhrij«صحيح : أخرجه البخاري ، كتاب الأدب : 6104 و مسلم : 60 و أبوداؤد : 4687 و الترمذي : 2637 - انظر الصحيحة : 2891»
Explanation & Benefits
Shaykh Maulana Usman Muneeb
Benefits and Issues: The point is that if the one being called a disbeliever (kafir) is truly such, then it is correct; otherwise, its burden will fall back upon the one making the accusation. For example, the Rafidi individuals say such things about the noble Companions (Sahabah radi Allahu anhum). Now, the Companions were, without any doubt, believers; therefore, those who make such accusations are themselves deemed disbelievers.
Source: Fadlullah al-Ahad: Urdu Commentary on al-Adab al-Mufrad, Page: 439