Numbers 18:25

“And the Lord spake unto Moses, etc.] And not unto Aaron, because a tithe out of the tithe was to be given to Aaron, as Aben Ezra observes, and it was not so proper that he should have this order to deliver to the Levites, in which he and his sons were so much concerned, and therefore it was given to Moses to acquaint them of it: saying; as follows.”

“18:25-32 Just as the Levites received a tithe of the Israelites’ goods (18:24), they in turn were required to give . . . a tithe of the tithe to the priests as their own harvest offering (18:27).”

“Appropriation of the Tithe. - Num 18:26. When the Levites took (received) from the people the tithe assigned them by Jehovah, they were to lift off from it a heave-offering for Jehovah, a tithe of the tithe for Aaron the priest (i.e., for the priesthood; see at Num 18:20). "Your heave-offering shall be reckoned to you as the corn of the threshing-floor, and the fulness (see Exo 22:28) of the wine-press," i.e., according to Num 18:30, as the revenue of the threshing-floor and wine-press; that is to say, as corn and wine which they had reaped themselves. Numbers 18:29”