Numbers 18:14-16

Every devoted thing in Israel is yours. The first offspring of the womb from all flesh, whether human or animal, offered to ADONAI, is yours. However, you are to redeem the firstborn of man and the firstborn of unclean animals. When they are a month old, you are to redeem them at the redemption price of five shekels of silver by the Sanctuary shekel, or 20 gerahs.