Tanach zu Nechemja 12:9

And of the sons of Aaron, their divisions: The sons of Aaron: Nadav and Avihu, Elazar and Ithamar.

And Nadav and Avihu died before their father. And they did not have sons, and Elazar and Ithamar served [(in their stead)].

And David divided them. And Tzaddok, of the sons of Elazar; and Achimelech, of the sons of Ithamar, were (appointed) by their numbers in their work.