What is the ruling on a girl saying to someone she loves: "Adam is forbidden to me after you"?

If a girl forbids herself from marrying anyone other than a specific person, this is an act of forbidding what is lawful, and it does not make unlawful for her what Allah has made lawful. However, jurists differ on whether an expiation for an oath (kaffarat yamin) is required of her or not. The more cautious approach is to offer expiation if she does what she had forbidden for herself. She must maintain chastity and avoid illicit relationships.