Numbers 18:3, 7

They shall attend to your needs and all the needs of the tabernacle; but they shall not come near the articles of the sanctuary and the altar, lest they die—they and you also.

Therefore you and your sons with you shall attend to your priesthood for everything at the altar and behind the veil; and you shall serve. I give your priesthood to you as a gift for service, but the outsider who comes near shall be put to death.”