I remember that a fast is due on me because of an oath I have taken. However, I do not remember on what I have sworn. Should I fast even without knowing what I swore about?

All perfect praise be to Allaah, The Lord of the Worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His Slave and Messenger.

If you are certain that you have taken an oath and then broken it, an oath expiation becomes due on you even without remembering the thing you have taken the oath on because what counts regarding the obligation to make an expiation is breaking a valid oath.

If you are certain that you have taken an oath which you have not broken, but you have forgotten the thing on which you have taken it, it is permissible for you to make expiation for it, depending on the opinion of the majority of scholars including Hanbali, Maaliki and Shaafi‘i scholars of Fiqh, which supports the permissibility to make expiation for an oath even before breaking it. By so doing, you will avoid the problem of forgetting the thing on which you have taken the oath. This expiation frees you of liability whether you have or have not broken your oath. You fear breaking it because you have forgotten the thing on which you have taken oath.

Here, it should be noted that fasting will not be sufficient for oath expiation unless one fails to feed or clothe ten needy persons or free a slave. If one is able to do any of those three, fasting will not be sufficient for him to fulfill the obligation.

Allaah Knows best.