Exodus 13:10-12 New King James Version (NKJV)

10. You shall therefore keep this ordinance in its season from year to year.

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

12. that you shall set apart to the Lord all that open the womb, that is, every firstborn that comes from an animal which you have; the males shall be the Lord’s.

Exodus 13