Zevachim, Chapter Nine, Mishnah 6. This mishnah deals with the law of temurah (substitution), and specifically with the law of the substitute for a Pesach offering.
The definition of temurah:
A person who takes an ordinary animal and declares that it should serve as a substitute for an animal that has already been consecrated as an offering - the animal that until that moment was not an offering now becomes an offering, and this does not remove the sanctity from the first animal. This act is forbidden by the Torah, yet if a person did it, it takes effect.
The words of Rabbi Yehoshua:
Rabbi Yehoshua said: "Shamati she'temuras Pesach kereivah u'temuras Pesach einah kereivah, ve'ein li lefaresh" - there is a case in which the substitute for a Pesach offering is brought on the altar, and there is a case in which the substitute for a Pesach offering is not brought, but he is unable to explain and distinguish between the two cases.
Rabbi Akiva's explanation:
Rabbi Akiva, his student, said: "I will explain." And he distinguishes between the two cases:
"HaPesach shenimtza kodem shechitas haPesach" - a person whose Pesach offering was lost, who set aside another in its place, and then found the first one before the replacement was slaughtered, and nevertheless slaughtered the replacement and not the original. The first one is not brought on the altar, because it falls under the category of something "actively rejected": he had the opportunity to offer it, since it was found before him before the slaughter, and he chose not to use it and rejected it with his own hands. Therefore: "Yir'eh ad sheyista'ev, ve'yimacher, ve'yikach bedamav shelamim, ve'chein temuraso" - it grazes until a blemish develops in it, it is sold (and this is the process of its redemption), and with the money he brings a shelamim offering. And the same law applies to the substitute of that rejected Pesach offering.
"Achar shechitas haPesach" - the original offering was found after the replacement was slaughtered. Here there is no active rejection, since it was not before him and he had no opportunity to use it. Therefore "kareiv shelamim" - the Pesach offering itself is brought as a shelamim, for this is the law of a Pesach offering whose opportunity to be offered has passed. "Ve'chein temuraso" - the substitute too is brought immediately as a shelamim.
The novelty of the mishnah:
The Gemara notes that this very same discussion could have been conducted regarding the Pesach offering itself, and there is no need to formulate it specifically in terms of temurah; at first glance the substitute seems to be a mere addition. The Gemara explains, however, that the novelty lies precisely in the temurah: one might have thought that in the first case, when the Pesach offering was found before the replacement was slaughtered, the substitute itself was never part of the problem at all and was never rejected, and therefore it itself would be offered as a shelamim rather than its proceeds. The mishnah comes to teach that this is not so: the substitute has the identical status as the original rejected Pesach offering - it grazes until a blemish develops in it, it is sold, and with the money he brings a shelamim.
In summary: In this mishnah we learned the words of Rabbi Yehoshua, who heard from his teachers that there is a substitute for a Pesach offering that is brought on the altar and one that is not brought, but did not know how to explain the distinction, and Rabbi Akiva's explanation: if the Pesach offering was found before the replacement was slaughtered, it is actively rejected, and it grazes until a blemish develops in it, it is sold, and with the money he brings a shelamim, and the same for its substitute; if it was found after the slaughter, there is no rejection here, and it itself is brought as a shelamim, and the same for its substitute. And the novelty of the mishnah is in equating the law of the substitute with the law of the Pesach offering itself.