Parashas Bamidbar · Passuk 3:45
Take the Levites in place of all the male first-born among the Israelite people, and the cattle of the Levites in place of their cattle; and the Levites shall be Mine, GOD’s.
Chabad.org — English translation + Rashi
Source: Bechoros 4b.
English: “In place of their animals” — this teaches that one sheep of a Levite exempts several firstborn donkeys of an Israelite.
In context: The exchange rate for the redemption of firstborn animals is derived from the parallel substitution of Levites and their livestock.
Custom
Chazal derive the rules of redeeming firstborn donkeys from the substitution language of this verse.