Numbers 18:3-6

3They 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.

4They 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.

5And you shall keep guard over the sanctuary and over the altar, that there may never again be wrath on the people of Israel.

6And 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.