What are the foundations for the expiation of an oath (Kaffarat al-Yamin)?

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The foundations for the expiation of an oath are the Book (Quran), the Sunnah, and the consensus (Ijma). The evidence from the Book is the verse which states the required expiations: feeding ten poor people from the average food provided to one's family, clothing them, or freeing a slave, and if one cannot afford that, then fasting for three days. This is stated in Quran 5:89. The evidence from the Sunnah is the Prophet's saying: If you swear an oath and see something else better than it, then do what is better and expiate your oath. Muslims are in consensus regarding the legislation of expiation for swearing by God Almighty.