Is expiation (kaffarah) obligatory for one who vowed not to look at a man, and then looked?

If you vowed to refrain from looking at men with lust, and then you did so, you owe an expiation for a broken oath (kaffarat yamin): feeding ten poor persons, or clothing them, or freeing a believing slave. If you are unable to do any of these three, then fast for three days. Making vows is disliked (makruh), but one must fulfill them if they involve obedience to Allah the Almighty.