whereas one who swears in anger must offer expiation for an oath.”
Moreover, what counts in a statement is its meaning, not its wording. This person’s intention in swearing is not to draw closer to Allah; he intends only to urge that an act be done or to prevent it. This is the very meaning of an oath, for one who swears intends to urge that something be done or to prevent it. When he swears by Allah Most High concerning that act, expiation suffices him. It should therefore suffice him all the more when he makes the obligation of an act of worship—or the prohibition of something permissible—contingent upon that act. For when he swears by Allah concerning it and then breaks his oath, the consequence of doing so is that he has violated the sanctity of oaths sworn by Allah by failing to honor his pledge. But when he makes the obligation or prohibition of an act contingent upon it, the consequence of breaking his oath is merely the omission of an obligatory act or the commission of a prohibited one. It is well known that a violation resulting in a defect in tawhīd (affirming Allah’s oneness) is graver than one resulting in a sin among other sins. Thus, if Allah has prescribed expiation to rectify and make good the damage that breaking an oath causes to tawhīd and similar matters, it is all the more fitting and proper that it be prescribed to rectify the damage it causes to obedience.
Moreover, we maintain that the legal effect of an oath formula is the same as that of a conditional formula. A vow is a type of oath, and every vow is an oath. Thus, the person making a vow who says, “I owe it to Allah to do this,” is like one who says, “I swear by Allah that I shall certainly do this.” Both statements have the same legal effect: committing oneself to perform the act and binding that commitment to Allah.
The evidence for this is the statement of the Prophet (peace and blessings be upon him): “A vow is an oath.” Thus, saying, “If I do such-and-such, I owe Allah a pilgrimage,” is equivalent to saying, “If I do such-and-such, then, by Allah, I shall certainly perform pilgrimage.”
A further implication of this principle is that if someone swears that he will perform a righteous act, he is obliged to perform it and may not offer expiation instead, for [his oath] that he will indeed perform it is a vow to do so.