1296 – Ibn Masʿūd (may Allah be pleased with him) reported that the Messenger of Allah ﷺ said, “Abusing a Muslim is moral deviance, and fighting him is disbelief.” Agreed upon.⦗1⦘

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Vocabulary of the ḥadīth
• sibāb: the verbal noun of sabba – yasubbu, pronounced with an initial short i and a light b. It denotes verbal abuse—speaking of someone in a manner that disgraces him. Ibrāhīm al-Harbī said: “sibāb is harsher than sabb; it is to say about a man what is in him and what is not.”
• fusūq: from fasaqa – yafsuqu, meaning licentiousness or departure from what is right. In this sentence it is the predicate, while the subject is “sibāb.”
• qitāluhu: “his fighting,” i.e., engaging in combat against him; this is the subject whose predicate is “kufr.”
• kufr: here it is not literal disbelief that expels one from Islam. The term is used for stern warning. A consensus among Ahl al-Sunnah affirms that a believer does not become an unbeliever merely by fighting another Muslim or by any other sin.

Lessons derived from the ḥadīth

1. Fusūq is departing from obedience to Allah into disobedience; insulting a fellow Muslim is one of the sins Allah has forbidden.
2. By implication the ḥadīth permits abusing a non-Muslim. Yet if that non-Muslim is under covenant, such abuse constitutes harm, which is prohibited. Therefore the implied allowance does not apply to him in light of other evidences and considerations.