Bikkurim, Chapter 2, Mishnah 6. This mishnah does not deal with the mitzvah of bikkurim at all; it was placed here because of what it shares with the preceding mishnayos in its style of construction: comparing certain items to other items, and listing the ways in which they are alike and the ways in which they differ.
The text of the mishnah:
"Esrog shaveh la'ilan bishloshah derachim v'layarak b'derech echad" - the esrog that we take on Sukkos resembles a tree and tree fruit in three matters, and resembles a vegetable in one matter of halachah.
The three ways in which the esrog is like a tree:
"B'orlah" - it may not be eaten during the first three years, because of the law of orlah.
"Uv'reva'i" - in the fourth year it is eaten in Jerusalem.
"Uvashevi'is" - with regard to all the laws of shemittah.
What does this mean? The halachah is that the esrog, like all other tree fruit, has its year determined by chanatah (the formation of the fruit). When the blossom falls off and a small bud of fruit becomes visible, that is chanatah, and the year in which it occurs is the determining year. If it formed in a given year, that becomes the first year, followed by the second and third for the laws of orlah, and the fourth for the laws of reva'i; and if it formed in the seventh year, it is subject to the laws of shemittah. In all of these it is like a tree, whose fruit is judged according to chanatah.
The one way in which the esrog is like a vegetable:
"Shebish'as lekitaso isuro" - with vegetables, the year is not determined by chanatah, but by the year in which they are picked and detached from the ground. So too with the esrog regarding its tithes: determining which tithe year it belongs to - whether a year of maaser sheni or a year of maaser ani - is done according to the time of picking, according to the year in which the esrog was picked and detached from the ground, exactly like the law of a vegetable.
All of this is "divrei Rabban Gamliel" - this is his opinion.
The opinion of Rabbi Eliezer:
"Rabbi Eliezer omer: shaveh la'ilan b'chol davar" - in his view the esrog resembles a tree in every respect, including the matter of the tithe year. This too is determined by its chanatah, and not by its picking and detachment from the ground.
In summary: this mishnah lists three matters in which the esrog is like a tree - orlah, reva'i and shevi'is, all of which are determined by chanatah - and one matter in which it is like a vegetable: the determination of the tithe year according to the time of picking. Rabbi Eliezer disagrees and holds that the esrog is like a tree in every respect, and that even its tithe year is determined by chanatah.