Numbers 18:3-5

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. They shall join you and keep guard over the tent of meeting for all the service of the tent, and no outsider shall come near you. And you shall keep guard over the sanctuary and over the altar, that there may never again be wrath on the people of Israel.