What is the ruling on saying "my religion is forbidden to me" in a moment of anger?

A Muslim must guard his tongue from slips, for the tongue is dangerous. It can cast its owner into the Fire if used in ways displeasing to Allah, or it can elevate him to the highest stations if used for good. The Prophet, peace and blessings be upon him, said: "Indeed, a servant may utter a word, not realizing its implications, that casts him into the Fire farther than the distance between the East and the West." (Reported by Muslim). Anger is among the most dangerous things that invite trouble and lead to ruin. The Prophet, peace and blessings be upon him, advised a man, saying: "Do not get angry." (Reported by Al-Bukhari and Muslim).

If the phrase "may your religion be forbidden to you" is intended to address another person with the intention of invoking death upon them while they are in a state of disbelief, then this is a grave major sin that could amount to apostasy if one intends to be pleased with their disbelief.

And if the intention is to address oneself by forbidding religion, then this is an abominable statement and its matter is serious, potentially leading its utterer to disbelief. However, the more correct view is that it does not constitute disbelief that expels one from the fold of Islam.

A Muslim must restrain his tongue from what is improper and beware of anger, and repentance obliterates what came before it.