Sefer HaChinuch ספר החינוך

To not eat the second tithe of oil outside of Jerusalem: To not eat the second tithe of oil outside of Jerusalem, as it stated (Deuteronomy 12:17), “You may not eat in your gates, etc. your oil.” And the whole content of oil is like the content of grain and wine. And the measurement of eating oil to make one liable for it is a kazayit, according to what I heard from my teacher, God protect him. And even though it is a drink, it is held to be eaten by all people. And [even] if we know a few Yishmaelites that drink it, their opinion is nullified by all the rest of the people.

And do not think to say that this negative commandment of “You may not” is a general prohibition, as each and every matter is a negative commandment on its own. And so is it explained in Tractate Makkot (it should say Keritot 4b), “If he ate tithe of grain, wine and oil, he is liable for each and every one.” And it challenges there, “And do we administer lashes for a general prohibition?” And it answers it, “The verse is superfluous. How is it? It is written (Deuteronomy 14:23), ‘And you shall eat in front of the Lord, your God [...] the tithe of your grain, your wine and your oil.’” [This discussion continues in Makkot 18a:] “Let the [Torah] write, ‘You may not eat them in your gates.’ Why do I need the [Torah] to enumerate all of them here? [Hence] we understand from it [that it is] to designate for them a negative commandment for each and every one.” And this verse of “And you shall eat” is at the end of this Order.

And in Sifrei Devarim 106, it is expounded that, that which this verse mentions “and the firstborn of your cattle and your flocks,” even though it is for the priests, is coming to compare the tithe to the firstborn, etc.