Numbers 18:5-7

And you shall keep guard over the sanctuary and over the altar, that there may never again be wrath on the people of Israel. And behold, I have taken your brothers the Levites from among the people of Israel. They are a gift to you, given to the LORD, to do the service of the tent of meeting. And you and your sons with you shall guard your priesthood for all that concerns the altar and that is within the veil; and you shall serve. I give your priesthood as a gift, and any outsider who comes near shall be put to death.”