Numbers 18:15-16

1512Every firstborn of the womb of all flesh, whether human or animal, which they offer to the Lord, shall be yours; however you must redeem the human firstborn, and the firstborn of unclean animals you shall redeem.

16As to 3their redemption price, from a month old you shall redeem them, by your assessment, five 4shekels in silver by the 5shekel of the sanctuary, which is twenty gerahs.