Numbers 18:15-16

15I give to you every firstborn animal and male child that people offer to the Lord. But you must pay the price for every firstborn son and for every firstborn male animal that is not clean.

16When they are one month old, you must buy them from me for five shekels of silver each. The shekel must have the same weight as the shekel that the Levites use in God's Holy Tent. There are 20 gerahs in one shekel.