What is the ruling on swearing falsely by Allah, and what must the swearer do to repent of his sin?

Lying is forbidden (haram), and its prohibition becomes even more severe if it is accompanied by an oath; this is what is called "al-yamin al-ghamūs" (the dive-in oath, or perjurious oath). Whoever swore a false oath must hasten to sincere repentance (tawbah nasūḥ) to Allah. This is the obligation according to the majority of scholars. Some of them went further, stating the obligation of expiation (kaffarah), which is: feeding ten poor persons, or clothing them, or freeing a slave. If one cannot find these, then fasting for three days.