What is the ruling on someone who says, "It is forbidden for me by my religion not to do such-and-such," and then does it? Is their religion thereby forbidden to them, and what expiation is necessary for that?

Whoever says, "It is forbidden by my religion," does not become an unbeliever nor does he exit his religion by breaking the oath. Rather, an expiation for an oath (kaffarat yamin) becomes obligatory upon him. He must also repent to Allah and train his tongue to swear by Allah Almighty, or by His beautiful names and sublime attributes.