Bikkurim, Chapter 3, Mishnah 9. This mishnah discusses the adornments of the bikkurim - the items added to decorate the bikkurim - and the question of how far they must conform to the laws of the bikkurim themselves.
Two opinions in the mishnah:
"Me'atrin es habikkurim chutz mishivas haminim" - according to the first opinion, one may adorn the bikkurim even with fruits that are not of the seven species.
Rabbi Akiva: "Ein me'atrin es habikkurim ela mishivas haminim" - Rabbi Akiva disagrees and holds that the adornment may be made only from the fruits of the seven species, exactly like the bikkurim themselves.
The adornment was done, as mentioned earlier, before they entered Jerusalem: they would beautify the bikkurim and decorate them with attractive fruits so that they would appear at their best. According to the first opinion, one could use for this purpose even apples, oranges and the like, which are not among the seven species.
Rabbi Akiva's reasoning:
The Yerushalmi explains that Rabbi Akiva made a decree, so that people would not come to err about the bikkurim themselves because of the adornment: if a person brings adornments for the bikkurim from species other than the seven species, onlookers might think that the bikkurim themselves are also brought from fruits that are not of the seven species.
For this very same reason, Rabbi Akiva would hold that the adornments of the bikkurim may be made only from the fruits of Eretz Yisrael, since Eretz Yisrael is the only place from which bikkurim are brought.
In summary: in this mishnah the Tannaim disagree about the law of the adornments of the bikkurim - according to the first opinion one may adorn them even with fruits that are not of the seven species, while according to Rabbi Akiva only with the seven species, out of concern that people will mistakenly say that the bikkurim themselves also come from other species. For that same reason, the adornments are required to be specifically from the fruits of Eretz Yisrael.