1 Chronicles 24:1-3

Now these are the divisions of the sons of Aaron. The sons of Aaron; Nadab, and Abihu, Eleazar, and Ithamar.

But Nadab and Abihu died before their father, and had no children: therefore Eleazar and Ithamar executed the priest’s office.

And David distributed them, both Zadok of the sons of Eleazar, and Ahimelech of the sons of Ithamar, according to their offices in their service.