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 18:17

18:17 But you must not redeem the firstborn of a cow or a sheep or a goat; they are holy. You must splash their blood on the altar and burn their fat for an offering made by fire for a pleasing aroma to the Lord.