Nazir, Chapter 6, Mishnah 1. The Mishnah opens by listing three categories of things that are forbidden to a nazir:
Tumah: He may not become tamei through contact with a corpse, with the body of the dead.
Shaving: He may not cut his hair.
What comes from the vine: Every item and every product that comes from the grape or from the vine is forbidden to him.
Combining the products of the vine with one another:
The Mishnah establishes that all the various things that come from the vine combine with one another to form a kezayis, which is the measure that makes a nazir liable to lashes for violating his nezirus. In other words, even if no single one of them contains the required measure on its own - a little of the skins, a little of the flesh of the grape and a little of the seeds - they all join together to make up that kezayis.
The measure of liability - a kezayis or a revi'is:
The measure of a kezayis, as mentioned, is the basis of the statement "eino chayav ad she'yochal min ha'anavim kezayis" - he is not liable until he eats a kezayis of the grapes. It is worth being precise: this idea, that a kezayis is the measure, is a later view in the time of the Mishnah. But the earlier Mishnah - the original opinion - held that a person is not liable until he drinks the measure of a revi'is of a log of wine, or until he eats an amount that reaches the volume of a revi'is.
The opinion of Rabbi Akiva:
Rabbi Akiva says: "afilu sharah pito b'yayin v'yesh bo k'dei l'tzaref kezayis - chayav" - even if he soaked his bread in wine and there is enough to combine to a kezayis, he is liable. That is, even if a person dipped his bread in wine, and the wine together with the bread combined to make up a kezayis, he is liable - and this is so even though there is no kezayis of wine on its own here, but only a kezayis when combined with the bread. The bread joins together with the wine.
This is a concept learned particularly in Maseches Nazir: the permitted item, the bread, combines with and is able to be included in the forbidden measure of a nazir drinking wine.
We find, then, that Rabbi Akiva teaches two new points:
He disagrees with the earlier Mishnah, which held that the measure for wine is a revi'is, and says that the measure is a kezayis.
Beyond that, he teaches that the permitted substance - the bread, which may be eaten - combines with the forbidden wine to make a person liable to lashes.