1 Chronicles 24:1

But Nadab and Abihu died before their father, and had no children; therefore Eleazar and Ithamar served as priests.

David, with Zadok of the sons of Eleazar and Ahimelech of the sons of Ithamar, divided them according to their ordering in their service.

There were more chief men found of the sons of Eleazar than of the sons of Ithamar; and they were divided like this: of the sons of Eleazar there were sixteen, heads of fathers’ houses; and of the sons of Ithamar, according to their fathers’ houses, eight.