Lev. 27; Numbers 1
The Israelites knew that God was due the first tenth of their increase whether it was in crops, land, or money. It belonged to God. Everything about God is holy, therefore the tithe was holy.
The principle of the tithe transcends the law. Abraham paid tithes to Melchizedek before the law was given. Hebrews 7 likens Melchizedek to Christ and establishes his ongoing priesthood, validating tithing in the New Testament.
God watches over that which is His.