What is the ruling on a person saying about another, "So-and-so declared him an infidel" out of exasperation with him, and is this considered an act of disbelief (kufr)? And what should a Muslim do if someone commits an act of disbelief in front of him: should he be told, "You have disbelieved, and you must re-enter Islam"? And is one excused for error, such as a slip of the tongue? And does the rule "Whoever does not declare an infidel to be an infidel is himself an infidel" apply in this case?

If a person says about another, "So-and-so declared me an unbeliever (kafara)," this is not an act of disbelief (kufr), whether he is truthful or lying. Ruling on a specific Muslim as being an unbeliever is contingent upon the fulfillment of conditions and the absence of impediments, and this judgment is the prerogative of the Sharia judiciary or the mujtahid scholars. The rule "Whoever does not declare an unbeliever to be an unbeliever is himself an unbeliever" applies to those who adhere to a religion other than Islam, and it has no relation to declaring someone who outwardly professes Islam to be an unbeliever if he falls into one of its nullifiers. If an individual commits an act of disbelief, you should advise him and clarify that this act is disbelief and that he must repent from it, rather than ruling him an apostate. The questioner is advised to ignore these whisperings (waswas) and not to dwell on them.