Deuteronomy 14:22

Thou shalt truly tithe. — The Talmud and Jewish interpreters in general are agreed in the view that the tithe mentioned in this passage, both here and in Deuteronomy 14:28, and also the tithe described in Deuteronomy 26:12, are all one thing — “the second tithe;” and entirely distinct from the ordinary tithe assigned to the Levites for their subsistence in Numbers 18:21, and by them tithed again for the priests (Numbers 18:26).

The tithe described in Numbers was called “the first tithe,” and was not considered sacred. The second tithe, on the contrary, was always regarded as a holy thing.