Numbers 18:5

2 And take with thee thy brethren also of the tribe of Levi, and the sceptre of thy father, and let them be ready in hand, and minister to thee: but thou and thy sons shall minister in the tabernacle of the testimony.

3 And the Levites shall watch to do thy commands, and about all the works of the tabernacle: only they shall not come nigh the vessels of the sanctuary nor the altar, lest both they die, and you also perish with them.

4 But let them be with thee, and watch in the charge of the tabernacle, and in all the ceremonies thereof. A stranger shall not join himself with you.

5 Watch ye in the charge of the sanctuary, and in the ministry of the altar: lest indignation rise upon the children of Israel.

6 I have given you your brethren the Levites from among the children of Israel, and have delivered them for a gift to the Lord, to serve in the ministries of the tabernacle.

7 But thou and thy sons look ye to the priesthood: and all things that pertain to the service of the altar, and that are within the veil, shall be executed by the priests. If any stranger shall approach, he shall be slain.

8 And the Lord said to Aaron: Behold I have given thee the charge of my firstfruits. All things that are sanctified by the children of Israel, I have delivered to thee and to thy sons for the priestly office, by everlasting ordinances.