then its expiation is to feed ten poor persons with the average of what you feed your own families, or to clothe them, or to free a slave. But whoever cannot afford this must fast for three days. That is the expiation for the oaths you have sworn. And safeguard your oaths.” [al-Ma'idah 5:89]
The command to safeguard them includes the command to offer expiation, because this is part of safeguarding them.
Likewise, the Prophet, peace and blessings be upon him, said: “If you swear an oath and then see that something else is better than it, offer expiation for your oath.” (1) Thus, he commanded expiation, and the basic principle regarding a command is that it denotes obligation.
His statement, “expiation”: this is derived from kafr, meaning concealment. The basic principle is that, when a person swears an oath, he is required to fulfil it, because he has sworn by something great. Part of its greatness is that one fulfils what one has sworn to do, for an oath is, in reality, an affirmation of something through the greatness of that by which one swears. If one violates this affirmation, it is as though one has indicated a violation of the greatness of that by which one swore. This is why expiation is obligatory. Had Allah Most High not shown mercy to His servants, whoever swore to do something would have been required to fulfil it in every circumstance, except in cases of necessity.
His statement, “The first: that the oath be binding,” meaning one that is established and confirmed. This is because the binding of something means that it is like a firm knot, fastening that which is tied by it. But its legal definition, as the author states, is: