Chapter on Vows
A vow is valid only if made by a sane adult, even if he is a disbeliever. Its valid forms are five categories.
The unrestricted vow is, for example, for someone to say, “I owe Allah a vow,” without specifying anything; in that case, he must offer the expiation for an oath.
His statement, “vow”: linguistically, it means making something obligatory. One says, “I have vowed this upon myself,” meaning: I have made it binding. In the Sacred Law, however, it is a particular kind of obligation: a legally accountable person binding himself to something possible that he owns.
A vow is established by verbal expression and has no fixed formula. Rather, anything indicating commitment constitutes a vow, whether one says, “I owe Allah a covenant,” “I owe Allah a vow,” or the like—anything that indicates commitment, such as, “I owe Allah that I do such-and-such”—even if he does not use the words “vow” or “covenant.”
The ruling on making a vow is that it is disliked. Indeed, Shaykh al-Islam Ibn Taymiyyah, may Allah have mercy on him, inclined to the view that it is prohibited, because the Prophet, peace and blessings be upon him, forbade vows and said, “It brings no good.” (1) Nor does it avert the divine decree: if Allah had willed to do something, He would have done it whether you made a vow or not. Since the Messenger, peace and blessings be upon him, prohibited it and explained that it has neither a religious benefit—for it brings no good—nor a benefit pertaining to the divine decree—for it does not avert the decree—the view that it is prohibited is strong.
It may be asked: How can Allah praise those who fulfil their vows when they have committed something prohibited?