Numbers 3:4-6

New Revised Standard Version, Anglicised Catholic Edition

⁴ Nadab and Abihu died before the Lord when they offered unholy fire before the Lord in the wilderness of Sinai, and they had no children. Eleazar and Ithamar served as priests in the lifetime of their father Aaron.

The Duties of the Levites

⁵ Then the Lord spoke to Moses, saying: ⁶ Bring the tribe of Levi near, and set them before Aaron the priest, so that they may assist him.