Pesachim Chapter 7, Mishnah 9. This mishnah deals with the various disqualifications that can befall the Pesach offering, and with the law of burning it: when the offering is burned immediately, and when we must wait until its form is left to decay.
"Pesach sheyatza o shenitma - yisaref miyad":
"Pesach sheyatza" - a Pesach offering that left the walls of Jerusalem. The Pesach may be eaten only within Jerusalem, and once it leaves the city its meat becomes disqualified from being eaten.
"O shenitma" - the Pesach remained in Jerusalem, but its meat became tamei.
In both of these cases the offering is burned immediately, because this is a disqualification in the body of the offering: the Pesach itself has become disqualified.
The owners became tamei or died:
However, if the owners of the Pesach became tamei and are unable to eat it, or if they died, we do not burn it immediately. Rather, "te'ubar tzurato" - we wait until its form and appearance become disqualified, meaning until it becomes notar and its time has passed, "veyisaref beshishah asar" - on the first day of Chol HaMoed, the sixteenth of Nissan.
It is worth noting the difference in timing: consecrated items are not burned on Yom Tov, and therefore in the first case, where the offering is burned immediately, we are speaking of a burning that takes place before Yom Tov. Here, however, where we must wait until its form decays and it becomes notar - something that happens only on the morning of the fifteenth of Nissan, which is still Yom Tov - the burning is pushed off to the morning of the sixteenth.
The opinion of Rabbi Yochanan ben Berokah:
Rabbi Yochanan ben Berokah disagrees and says: "Af zeh yisaref miyad" - even in a case where the owners became tamei or died, the offering is burned immediately, "lefi she'ein lo ochlin", since no one is left to eat it.
The reason: eating the Pesach offering is one of its essential features. Unlike other offerings, whose consumption may serve as atonement but does not constitute the main mitzvah in them, the Pesach offering is entirely about being eaten. For that reason, when there is no one to eat it, Rabbi Yochanan ben Berokah views this as a disqualification in the body of the offering, as though the Pesach offering itself had become disqualified.
How the Gemara limits the dispute:
The Gemara explains that Rabbi Yochanan ben Berokah disagrees only when the owners became tamei or died before the blood of the Pesach offering was sprinkled. Since the meat of the offering could never have become fit to be eaten, it is as though the offering itself became disqualified. But if the sprinkling had already been performed, and only afterward the owners became tamei or died, this is not considered a disqualification in the body of the offering, and even Rabbi Yochanan ben Berokah agrees that it is not burned immediately. Rather, we wait until it becomes notar and burn it on the sixteenth of Nissan.
In summary: This mishnah distinguishes between a disqualification that occurred in the body of the offering - leaving the walls of Jerusalem or the meat becoming tamei, in which case it is burned immediately - and a disqualification that occurred in the owners, who became tamei or died, in which case we wait until its form decays and it is burned on the sixteenth of Nissan. Rabbi Yochanan ben Berokah holds that a disqualification in the owners is also considered a disqualification in the body of the offering, since the essence of the Pesach offering is its being eaten, and the Gemara limits his dispute to a case where this occurred before the sprinkling of the blood.