Is expiation obligatory upon one who swore by Allah falsely out of ignorance, and then later came to know of it?

If you intentionally swore falsely by Allah after reaching puberty, you must repent to Allah, for this is one of the grave sins, known as al-Yameen al-Ghamoos (the

Perjurious Oath), and its expiation is repentance. If you offer an expiation for an oath, that is more cautious. However, if you swore about something that was not true without intending to lie, this falls under Laghw al-Yameen (a vain or unintentional oath), and there is no sin or expiation for it.