Shevi'is, Chapter 8, Mishnah 10. In the previous mishnah we dealt with the opinion of Rabbi Eliezer, that if a hide was rubbed with oil of Shevi'is, the hide itself must be burned. The Sages disagreed and held that this is not necessary, and Rabbi Akiva refrained from revealing to the students and the Sages what he had heard from Rabbi Eliezer on the matter. The Yerushalmi records a dispute about how to understand this: some understood that Rabbi Eliezer was extremely lenient, and some understood that he was extremely strict.
Something similar appears in our mishnah, except that the subject is different:
The text of the mishnah:
"Ve'od amru lefanav: omer hayah Rabbi Eliezer, ha'ochel pas Kusim ke'ochel basar chazir" - they said before Rabbi Akiva that in Rabbi Eliezer's opinion, one who eats bread baked by Kusim is as though he ate the flesh of a pig. That is how severely he viewed the matter.
Who were the Kusim:
The Kusim were a nation settled in Eretz Yisrael by the king of Assyria toward the end of the First Temple period, and they converted upon their arrival in the land. The Gemara records a dispute as to whether their conversion was a genuine one or not, that is, whether they were "converts out of fear of the lions" or genuine converts. In any case, by the end of the mishnaic period and in the time of the Gemara it was already established that their status is that of gentiles, once it became clear that they were actual idol worshippers.
Their halachic status in the words of the mishnah:
The Mishnah generally deals with the Kusim during the period when they were still considered Jews, and at times it follows the view that they are Jews and at times the view that they are not. Our mishnah follows the position that they are not genuine converts and are not considered Jews in every respect, and therefore the Sages decreed a prohibition against eating their bread, just like the bread of a gentile.
Regarding the bread of a gentile itself the halachic authorities disagree, and most authorities are lenient when it is bread baked by a non-Jewish professional baker, as opposed to the bread of a private person baking his own loaf, concerning which one should be strict in most situations.
Rabbi Akiva's response:
"Shetuku, lo omar lachem mah she'Rabbi Eliezer omer bo" - be silent, I do not intend to reveal to you Rabbi Eliezer's opinion on this matter. The commentators explain that Rabbi Eliezer was in fact extremely lenient regarding the bread of those Kusim and permitted it, and Rabbi Akiva did not want to say anything before them that contradicted the tradition they held and what they understood to be Rabbi Eliezer's opinion, and so he left matters as they were.
In summary: in this mishnah we learned Rabbi Eliezer's opinion that one who eats the bread of Kusim is as one who eats the flesh of a pig, and the basis of the prohibition, namely that the Kusim are people whose conversion was not genuine, so that the law of their bread is like the law of a gentile's bread. We also examined Rabbi Akiva's response, in which he refrained from revealing Rabbi Eliezer's true opinion, that he was lenient in the matter, so as not to contradict the tradition held by his listeners.