Numbers 18:14-16

“You get every Totally-Devoted gift. Every firstborn that is offered to GOD, whether animal or person, is yours. Except you don’t get the firstborn itself, but its redemption price; firstborn humans and ritually unclean animals are bought back and you get the redemption price. When the firstborn is a month old it must be redeemed at the redemption price of five shekels of silver, using the standard of the Sanctuary shekel, which weighs twenty gerahs.