Yoma, Chapter 8, Mishnah 2. This mishnah deals with shiurim - the quantities required in order to be liable to kareis on Yom Kippur.
The text of the mishnah:
"Ha'ochel k'choveses hagassah, kamoha v'chagar'inasah, v'hashoseh melo lugmav - chayav" - one who eats the volume of a large date, the date itself together with its pit, and one who drinks a cheekful, is liable. Here are the details:
"K'choveses hagassah" - the volume of a large date.
"Kamoha v'chagar'inasah" - the flesh of the date itself along with its pit, that is, the whole date together with the pit.
"Melo lugmav" - the fill of one cheek: the amount of liquid that would fill one side of his mouth were he to push it over to that side.
And what is the liability? One who does this intentionally is liable to kareis, and one who does it unintentionally must bring a chatas offering.
How these measures compare to the other measures in the Torah:
The measure of a large date is slightly larger than a kezayis, and slightly smaller than a kebeitzah, which is double a kezayis. It therefore falls midway between the two. The measure for drinking is gauged according to each individual person, and for an average person it is small - less than a revi'is.
Why do these measures differ from all other cases of eating and drinking in the Torah, where drinking is measured by a revi'is and eating by a kezayis? Because here the Torah did not define the prohibition in terms of eating, but in terms of "ve'inisem es nafshoseichem" - "and you shall afflict your souls," and these are the amounts at which a person no longer feels afflicted.
Combining the measures:
The mishnah teaches that one need not eat a single type of food to complete the amount that renders him liable to kareis; rather, all the various kinds of food combine together to make up the measure of a large date, and likewise all the various kinds of beverages combine together to make up a cheekful.
Eating and drinking, however, do not combine with one another: they are two separate measures. Therefore, one who ate half the measure of eating and drank half the measure of drinking - these do not combine to render him liable to kareis or to a chatas offering.
In summary: in this mishnah we learned the measures of liability on Yom Kippur - a large date together with its pit for eating, and a cheekful for drinking; where they stand relative to the measures of a kezayis and a revi'is used throughout the Torah, and the reason they differ, which is derived from the expression "ve'inisem es nafshoseichem"; and also the law of combining: foods combine with one another and beverages combine with one another, but eating and drinking do not combine with each other.