Exodus 13:11-13

11“Now it shall be when the Lord brings you into the land of the Canaanite, as He swore to you and your fathers, and gives it to you,

12you shall set apart and dedicate to the Lord all that first opens the womb. All the firstborn males of your livestock shall be the Lord’s.

13Every 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].