Read Exodus 13:12 Using Other Translations
That thou shalt set apart unto the LORD all that openeth the matrix, and every firstling that cometh of a beast which thou hast; the males shall be the LORD'S.
you shall set apart to the LORD all that first opens the womb. All the firstborn of your animals that are males shall be the LORD's.
you must present all firstborn sons and firstborn male animals to the LORD, for they belong to him.