Is it permissible to insult a disbeliever?

Hostile disbelievers have no sanctity. As for the dhimmi (protected non-Muslims) and the mu'ahid (those with whom a treaty has been made), their honor is protected, their blood is inviolable, and their property is guaranteed. Harming them is forbidden. A Muslim insulting a dhimmi is a sin, and the perpetrator should be disciplined, whether the dhimmi is alive or has died upon disbelief. The understanding of the hadith (definition), "Insulting a Muslim is a transgression (fusuq)," implies the permissibility of insulting a disbeliever. However, if the disbeliever is a mu'ahid, then insulting them is a harm that is prohibited. As for the hostile disbeliever, it is permissible to insult them, as they have no sanctity. A Muslim must guard their tongue from indecency and vulgarity, even with non-Muslims, as was narrated concerning the Prophet, peace and blessings be upon him, forbidding Aisha from using vulgar language in her response to the Jews.