The 129th mitzvah is that the Levites are commanded to give the kohanim ma'aser (one-tenth) from the ma'aser they themselves get from the Israelites.

The source of this commandment is G‑d's statement,1 "Speak to the Levites and say to them: 'When you take from the Israelites the ma'aser that I have given you as your inheritance from them, you must separate from it G‑d's terumah, ma'aser from the ma'aser' "

The Torah explains that this ma'aser, which is called terumas ma'aser, is given to the Kohen. This is seen from the verse,2 "And you must give it as G‑d's terumah to Aharon the Kohen."

The Torah also explains that this ma'aser must be given from the choicest and the best, as the verse3 says, "[You must separate G‑d's terumah] from the choicest as its dedicated portion."

[In the next few paragraphs, the Rambam addresses the prohibition against giving terumas ma'aser from inferior produce. He proves that although this appears to count as a separate prohibition, in actuality it does not.]

The Torah then points out that it is a sin not to take from the choice portion, as the verse4 says, "You do not bear any sin if you separate from the choicest." This is an implied prohibition — saying "you won't get a sin if you separate from the best" as a way of implying that it is a sin if you give from inferior produce.

This statement is therefore considered like a lav haba miklal aseh (a prohibition derived from a positive command), which is not counted as a separate prohibition.5 It is just that since one is commanded to give from the choicest, one automatically learns that the opposite is forbidden.

In the words of the Sifri: "What is the source of the law that one who gives from the inferior [produce] is committing a sin? From the verse, 'You do not bear any sin if you separate from the choicest.' "

The details of this mitzvah are explained in tractates Terumos, Ma'aseros, and a number of passages in Demai.