You shall also associate with yourself your kinsmen the tribe of Levi, your ancestral tribe, to be attached to you and to minister to you, while you and your sons under your charge are before the Tent of the Pact. They shall discharge their duties to you and to the Tent as a whole, but they must not have any contact with the furnishings of the Shrine or with the altar, lest both they and you die. They shall be attached to you and discharge the duties of the Tent of Meeting, all the service of the Tent; but no outsider shall intrude upon you as you discharge the duties connected with the Shrine and the altar, that wrath may not again strike the Israelites.
Numbers 18:2-5