Num 18:15-16

15The first issue of the womb of all creatures, human and animal, which is offered to the Lord, shall be yours; but the firstborn of human beings you shall redeem, and the firstborn of unclean animals you shall redeem.16Their redemption price, reckoned from one month of age, you shall fix at five shekels of silver, according to the shekel of the sanctuary (that is, twenty gerahs).