Offerings and their values, tithes: Leviticus 27
This is the first command for tithing. Previously, God had said it was all his anyway, so the concept of giving Him 10 per cent of produce would not have been outrageous.
27:30-33 One-tenth of the produce of the land, whether grain from the fields or fruit from the trees, belongs to the Lord and must be set apart to him as holy. … Count off every tenth animal from your herds and flocks and set them apart for the Lord as holy. You may not pick and choose between good and bad animals, and you may not substitute one for another … both the original animal and its substitute will be considered holy and cannot be bought back.