Numbers 18:3, 6-7

They shall keep guard over you and over the whole tent, but shall not come near to the vessels of the sanctuary or to the altar lest they, and you, die.

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.”