Numbers 3:4-6

Amplified Bible

⁴ But Nadab and Abihu died in the presence of the Lord when they offered strange (unholy, unacceptable, inappropriate) fire before the Lord in the Wilderness of Sinai; and they had no sons. So Eleazar and Ithamar served as priests ^[a]in the presence and under the supervision of Aaron their father.^(A)

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