Kilayim, Chapter 7, Mishnah 5. This mishnah is really nothing more than an illustration of the ruling of the previous mishnah, and indeed in some versions it is counted as part of the previous mishnah itself.
The text of the mishnah:
"Amar Rabbi Yosei" - Rabbi Yosei comes to support the position he stated in the previous mishnah, that a person cannot prohibit something that does not belong to him: with regard to kilayim, a person cannot create a prohibition in his fellow's field and in what grows there.
"Ma'aseh b'echad shezara es karmo bashevi'is" - a man sowed grain in his own vineyard during the Shemittah year.
"Uva ma'aseh lifnei Rabbi Akiva, v'amar: ein adam mekadesh davar she'eino shelo" - the case was brought for judgment before Rabbi Akiva, and he ruled that a person does not cause a prohibition in something that does not belong to him.
At first glance this is puzzling: the vineyard is his own vineyard, and he is planting in what belongs to him. The answer is that the planting was done in the Shemittah year, and during the seventh year a person's vineyard is considered ownerless and does not belong to him. Since at that moment the vineyard is not his, the rule applies: a person cannot prohibit something that is not his.
This case is the proof that Rabbi Yosei brought for his position in the previous mishnah - that with regard to kilayim as well we apply the rule that a person does not create a prohibition in something that does not belong to him, and during Shemittah the vine does not belong to him.
The comment of the Rosh, cited by Rabbi Akiva Eiger:
Even though the grapes themselves do not become forbidden, since they are not his - for they are ownerless and anyone may take possession of them - the vines themselves are not ownerless, only their fruit. Therefore, if these vines were to grow further on account of the kilayim by at least one two-hundredth - which is the minimum measure, any additional growth less than that being nullified - a prohibition would indeed be created here, since the vines are not ownerless, and only the grapes, the fruit, are ownerless.
In summary: in this mishnah Rabbi Yosei brought a case that came before Rabbi Akiva, concerning one who sowed his vineyard during Shemittah, from which he derived his position that a person cannot prohibit something that is not his - for during Shemittah the vineyard is ownerless and is not his. We also learned the comment of the Rosh: the ownerless status applies to the fruit alone, and therefore if the vines themselves grow by one two-hundredth on account of the kilayim, a prohibition is created.