Numbers 3:2

3:2 These are the names of the sons of Aaron: Nadab, the firstborn, and Abihu, Eleazar, and Ithamar.

Numbers 3:42

3:42 So Moses numbered all the firstborn males among the Israelites, as the Lord had commanded him.

Numbers 3:46

3:46 And for the redemption of the 273 firstborn males of the Israelites who exceed the number of the Levites,

Numbers 26:5

Reuben

26:5 Reuben was the firstborn of Israel. The Reubenites: from ¹  Hanoch, the family of the Hanochites; from Pallu, the family of the Palluites;

¹ tc The Hebrew text has no preposition here, but one has been supplied in the translation for clarity. Cf. vv. 23, 30, 31, 32.