What are the consequences for someone who says, "I bear witness before Allah that I will not commit such-and-such a sin until the Day of Resurrection," and then commits that sin?

Whoever commits this sin must repent and not return to it.

Scholars have differed on the obligation of expiation (kaffarah). Some consider it absolutely obligatory, some do not consider it obligatory, and some differentiate between the intention of making an oath and lacking it.

The most balanced opinion is that expiation becomes obligatory if the speaker intended an oath by saying, "I call upon Allah to witness."

If he intended it, then he must perform the expiation for an oath: feeding ten poor persons, or clothing them, or freeing a slave. If he is unable to do so, then fasting for three consecutive days is more cautious. If he did not intend an oath, then there is nothing upon him.