Numbers 18:6-7

Behold, I Myself have taken your brethren the Levites from among the children of Israel; they are a gift to you, given by the LORD, to do the work of the tabernacle of meeting. Therefore you and your sons with you shall attend to your priesthood for everything at the altar and behind the veil; and you shall serve. I give your priesthood to you as a gift for service, but the outsider who comes near shall be put to death.”