הלכה על במדבר 18:29

The commandment of the Levites to give a tithe from the tithe: That the Levites were commanded to separate a tithe from the tithe they take from Israel and that they give it to the priests, as it is stated (Numbers 18:26), "And to the Levites you shall speak, etc. and they shall give from it a gift to the Lord, a tithe from the tithe." And this is what is called 'terumat ma'aser' (literally, the gift of the tithe) in every place in the Gemara. They called it this expression in the way of the verse, which states, "a gift to the Lord." But the verse elucidates that he gives it to the priest, and as it states (Numbers 18:28), "and you shall give from it, the gift of the Lord, to Aharon the priest." And Scripture warns that he separate from the good and the choicest from the tithe, as it states (Numbers 18:29), "from all the fat that you sanctify from it." Moreover it is stated about this (Numbers 18:32), "You shall bear no sin upon it in separating its fat from it" - teaching that if he takes it out from the inferior [portion], there will be a sin upon them. And this matter is like the matter of a negative commandment that comes by way of deduction from a positive commandment, and therefore it is not to be counted among the negative commandments.