Numbers 3:9-11

Amplified Bible

⁹ You shall give the Levites to Aaron and to his sons [as servants and helpers]; they are to be given wholly to him from among the Israelites. ¹⁰ So you shall appoint Aaron and his sons, and they shall observe and attend to their priesthood; but the layman (non-Levite) who approaches [the holy things] shall be put to death.”

¹¹ Again the Lord spoke to Moses, saying,