Exodus 13:13-14

Every firstborn of a donkey you shall redeem by [substituting] a lamb [as a sacrifice for it], but if you do not [wish to] redeem it, then you shall break its neck; and every firstborn among your sons you shall redeem [that is, ‘buy back’ from God with a suitable sacrifice]. And it shall be when your son asks you in time to come, saying, ‘What does this mean?’ you shall say to him, ‘With a strong and powerful hand the LORD brought us out of Egypt, from the house of bondage and slavery.