What is the ruling of Sharia on vows?

A vow (nadhr) is when a legally competent individual obligates himself to something he was not previously bound by. Fulfilling a legitimate vow is obligatory. Allah has praised those who fulfill their vows in the Holy Quran. However, Prophetic traditions (Sunnah) have prohibited vows because a vow does not change fate in any way and is used to extract something from a miserly person.

The difference between a praised vow and a forbidden one: A praised vow: This is an unconditional vow of obedience, such as giving thanks for a blessing. A forbidden vow: This is a conditional vow of exchange, where the person making the vow makes his act of obedience conditional upon the occurrence or prevention of something. The wisdom behind prohibiting it is that the one making the vow performs the act of devotion reluctantly, and it resembles an exchange, which mars the intention, or a pre-Islamic belief that a vow can avert divine decree.

Types of vows regarding the obligation to fulfill them: 1. Vow that must be fulfilled (vow of obedience): This is any vow made in obedience to Allah, such as vowing to pray or fast. This includes vows conditional on a specific event, such as saying: "If my patient is cured, I owe a certain number of days of fasting." If one is unable to fulfill it, then he must offer expiation for an oath (kaffarat yamin). 2. Vow that must not be fulfilled and requires expiation for an oath: A vow of disobedience: Such as a vow made to other than Allah or to commit a sin. This requires expiation for an oath. Any vow that contradicts a religious text: Such as vowing to fast on the day of Eid al-Adha (Nahr). This requires expiation for an oath. 3. Vow that has no ruling other than expiation for an oath: An absolute vow where the object of the vow is not specified, such as saying: "I owe a vow." 4. Vow where there is a choice between fulfillment or expiation for an oath: A vow of contention and anger (nadhr al-lajaj wal-ghadab): This is a vow made with the intention of urging or preventing something, not for seeking closeness to Allah. In this case, one has a choice between fulfilling it or offering expiation for an oath. A vow of permissible acts (nadhr al-mubah): Such as vowing to eat a specific food. It is permissible to either fulfill it or offer expiation, and it should be fulfilled if it does not involve polytheism or disobedience.