15 The first offspring of every womb, both human and animal, that is offered to the LORD is yours. But you must redeem every firstborn son and every firstborn male of unclean animals.

Read Numbers 18:15 Using Other Translations

Every thing that openeth the matrix in all flesh, which they bring unto the LORD, whether it be of men or beasts, shall be thine: nevertheless the firstborn of man shalt thou surely redeem, and the firstling of unclean beasts shalt thou redeem.
Everything that opens the womb of all flesh, whether man or beast, which they offer to the LORD, shall be yours. Nevertheless, the firstborn of man you shall redeem, and the firstborn of unclean animals you shall redeem.
“The firstborn of every mother, whether human or animal, that is offered to the LORD will be yours. But you must always redeem your firstborn sons and the firstborn of ceremonially unclean animals.