Leviticus 27:32-34

³²And every tithing of oxen or sheep, whatever passeth under the tithing rod, shall be sacred before the Lord.

³³He shall not choose between the good and bad, neither shall he change it. But if he will commute it, both it and that for which it was exchanged shall be consecrate, and not be redeemed.

³⁴These are the commandments which the Lord commanded Mosheh for the children of Israel, in the mountain of Sinai.