Numbers 1:46

These are those that were numbered, which Moses and Aaron numbered, and the princes of Israel, being twelve men: each one was for the house of his fathers.

So were all those that were numbered of the children of Israel, by the house of their fathers, from twenty years old and upward, all that were able to go forth to war in Israel;

But the Levites after the tribe of their fathers were not numbered among them.

For the LORD had spoken unto Moses, saying,