Numbers 3:10-12

Amplified Bible

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, ¹² “Behold, I have taken the Levites from among the sons of Israel instead of and as a substitute for every firstborn, that is, the first that opens the womb among the Israelites. So the Levites shall be Mine,