Numbers 18:15-16

15“Everything that first opens the womb of all flesh, which they bring to the Lord, whether man or beast, shall be yours; nevertheless the firstborn of man you shall surely redeem, and the firstborn of unclean animals you shall redeem.

16And those redeemed of the devoted things you shall redeem when one month old, according to your valuation, for five shekels of silver, according to the shekel of the sanctuary, which is twenty gerahs.