Numbers 18:15

18:15 The firstborn of every womb which they present to the Lord, whether human or animal, will be yours. Nevertheless, the firstborn sons you must redeem, and the firstborn males of unclean animals you must redeem.

Exodus 13:13

13:13 Every firstling of a donkey you must redeem with a lamb, and if you do not redeem it, then you must break its neck. Every firstborn of your sons you must redeem.