Numbers 3:4

And Nadab and Abihu died before the Lord when they offered strange fire before the Lord, in the Wilderness of Sinai, Leviticus 10:1; and they had no children. And Eleazar and Ithamar ministered in the priest's office in the sight of Aaron, their father, that is, during his lifetime. The two men whom the wrath of God had struck down had no children, and therefore the two youngest sons of Aaron alone performed the work of the priests' ministry.