Is expiation (kaffarah) required for someone who said "yes" in response to "By God?" while being untruthful?

If your saying "yes" was intended as an oath and a confirmation of the questioner's statement: "By Allah," then it is a clear oath. If it was a lie, then it is a perjury (yamin ghamus), and its expiation, according to the majority of scholars, is sincere repentance and seeking forgiveness. However, it is more cautious to offer the expiation for an oath. But if the statement came from your tongue without intending an oath, then it is a vain utterance (laghw) and the rulings of an oath do not apply to it.