Numbers 1:20

1:20 And they were as follows:

The descendants of Reuben, the firstborn son of Israel: According to the records of their clans and families, all the males twenty years old or older who could serve in the army were listed by name individually.

Numbers 8:16

8:16 For they are entirely given to me from among the Israelites. I have taken them for myself instead of all who open the womb, the firstborn sons of all the Israelites.