Numbers 18:15-16
15 The firstborn of every living thing, man or animal, presented to the Lord belongs to you. But you must certainly redeem the firstborn of man,(A) and redeem the firstborn of an unclean animal.
16 You will pay the redemption price for a month-old male according to your assessment: five shekels of silver by the standard sanctuary shekel, which is 20 gerahs.(B)