Numbers 18:15-16

15The firstborn of every womb which they present to the Lord, whether human or animal, will be yours. Nevertheless, the firstborn sons you must redeem, and the firstborn males of unclean animals you must redeem.

16And those that must be redeemed you are to redeem when they are a month old, according to your estimation, for five shekels of silver according to the sanctuary shekel (which is twenty gerahs).