[Explanation]

Al-Nawawī—may Allah have mercy on him—states in his book Riyāḍ al-Ṣāliḥīn, under “The Prohibition of Abusing a Muslim Without Right,” that Abu Dharr (may Allah be pleased with him) related: the Prophet ﷺ said, “If a man calls his brother a disbeliever or a sinner, the description will rebound upon him unless his brother is truly as he claims.” In other words, if you say to someone, “You are a fāsiq,” or address him, “O fāsiq,” you yourself become the fāsiq unless he really is one; and the same applies to whoever brands another a kāfir, saying, “You are a kāfir,” or, “O kāfir,” when that person is not in fact a disbeliever—such a speaker becomes the kāfir. This shows that the act ranks among the major sins, for the Prophet ﷺ threatened that the accuser would bear the very trait he tried to pin on another. Accordingly, it is unlawful for a Muslim to say to his believing brother, “O sinner,” or state, “So-and-so is a sinner,” unless it is genuinely the case and he intends to warn others; in that circumstance there is no blame. Likewise, he must not say, “O disbeliever,” or declare, “So-and-so is a disbeliever,” for such speech is impermissible unless the charge is proven. The ḥadith therefore warns against takfīr—declaring Muslims unbelievers—without sound Sharīʿah-based evidence, unlike the rashness of certain people, may Allah protect us, who pronounce disbelief or sinfulness over the slightest matter, saying, “This is disbelief, that is sin,” and the like.

As for the second ḥadith, it is narrated from Abu Hurayrah (may Allah be pleased with him) that the Prophet ﷺ said: “When two people trade insults, the burden of what they say rests upon the one who began…”

The expression al-mutasābān (“the two who exchange curses”) is the first subject; mā (“whatever”) functions as a second subject; fa ʿalā al-bādiʾ (“then upon the one who began”) is the predicate of the second subject; and the entire clause serves as the predicate of the first. The meaning is that when two people hurl abuse at one another with foul words, the sin of everything uttered is borne by the one who initiated the exchange.