It is narrated from Sayyiduna Abdullah (may Allah be pleased with him) that it is reported from the Prophet (peace and blessings of Allah be upon him): "Abusing a Muslim is a sin, and fighting him is disbelief."
Hadith Referenceالادب المفرد / كتاب السباب / 431
Hadith Gradingالألبانی:صحيح
Hadith Takhrij«صحيح : أخرجه البخاري ، كتاب الإيمان ، باب خوف المومن من أن يحبط عمله ........ : 48 ، 6044 ، 7076 و مسلم : 64 و النسائي : 4111 و الترمذي : 1983 و ابن ماجه : 69»
Explanation & Benefits
Shaykh Maulana Usman Muneeb
Benefits and Issues: ➊ To abuse a Muslim is a sin, and if his honor is violated, its severity increases even more. In one hadith, this has been described as the greatest form of usury (riba), while the least degree of the sin of usury is likened to committing adultery with one's own mother. ➋ If someone considers fighting (qital) with a Muslim to be lawful (halal), then he is truly a disbeliever (kafir). However, if both parties believe themselves to be in the right, then that is a different matter. Or, it may be that the term "disbelief" (kufr) here is used for emphasis, meaning that one should avoid fighting with a Muslim, because it can also become a cause of disbelief (kufr).
Source: Fadlullah al-Ahad: Urdu Commentary on al-Adab al-Mufrad, Page: 431