Numbers 18:15-16

15The firstborn of every womb, whether man or beast, that is offered to the LORD belongs to you. But you must surely redeem every firstborn son and every firstborn male of unclean animals. 16You are to pay the redemption price for a month-old male according to your valuation: five shekels of silver,1 according to the sanctuary shekel, which is twenty gerahs.2