What is the ruling on vowing to perform an already obligatory act (e.g., a required prayer)?
General Chapter
Al-Mughni
Book of Vows
Primary text
The vow concerning an act that is already obligatory, such as a prescribed prayer, is invalid. This is because a vow is a commitment, and one cannot validly commit to something already required of them. Therefore, no expiation is due if the person neglects the act, similar to swearing an oath regarding something already obligatory.
Supporting text
There is a potential alternative view that the vow might be considered valid, requiring the expiation of an oath if abandoned, based on the analogy that a vow functions like an oath.