Pesachim, Chapter 3, Mishnah 8. In the previous Mishnah we discussed a person who set out on a journey for the sake of a mitzvah and then remembered that he had chametz at home, and we examined case by case whether he must return and destroy the chametz or whether nullification is enough. Our Mishnah continues along the same line and discusses a parallel case involving sacrificial meat.
The case in the Mishnah:
"Vechein mi sheyatza mi'Yerushalayim venizkar sheyesh beyado besar kodesh" - a person left Jerusalem, and after he had already left he remembered that he was carrying meat of a sacrifice. For example, meat of a shelamim offering, which may be eaten anywhere within Jerusalem but may not be eaten outside the city, and he left while holding this meat. As a rule, when part of a sacrifice becomes disqualified it must be destroyed and burned, and the mitzvah is to burn it in the Beis HaMikdash. The question, then, is whether he must return and burn it in the Beis HaMikdash or not.
"Im avar es Tzofim - sorfah bimkomah":
The Mishnah answers that it depends on the distance. 'Tzofim' is the place we call Har HaTzofim, in English Mount Scopus, and its name comes from the word for looking out - a lookout point from which the Temple Mount can be seen. This is where the old Hadassah hospital stands. If the person has already passed this point and moved farther away, he burns the meat wherever he is.
"Im lav - chozer vesorfah lifnei habirah be'atzei hama'arachah":
If he is not that far away and is still close to Jerusalem, he returns and burns the meat before the birah, meaning in the Beis HaMikdash. In the Gemara in Maseches Yoma there is a dispute as to what the birah is: Rabbi Yochanan says it is a particular place on the Temple Mount, and Reish Lakish says that the entire Beis HaMikdash is called the birah. Either way, the burning is done "be'atzei hama'arachah" - with the wood designated for the arrangement to be burned upon the altar, and he is permitted to use it.
"Vechamah hen chesran?" - the measure that requires returning:
What is the amount that obligates a person to return, both for chametz and for the sacrificial meat in his possession?
Rabbi Meir says: "Zeh vazeh bekebeitzah" - for both of them the measure is the size of an egg.
Rabbi Yehudah says: "Zeh vazeh bekezayis" - for both of them the measure is a kezayis.
And the Sages say: "Besar kodesh bekezayis" - for sacrificial meat the smaller measure was set, a kezayis, while for chametz the larger measure applies, the size of an egg.
The basis of these opinions: the measure of an egg is the measure we use for the tumah of foods - a piece of food transmits tumah to something else once it contains the volume of an egg, and Rabbi Meir adopts this criterion. Rabbi Yehudah holds that the prohibition of eating a forbidden substance applies at a kezayis, and therefore we follow that measure.
The Sages split the difference: sacrificial meat is measured by the stricter amount of a kezayis because of the special stringency that applies to the meat of sacrifices, which is treated more severely in many ways in Torah law. For this reason the Sages were stringent with sacrificial meat, requiring him to return even for a kezayis, even though it does not amount to the volume of an egg.
In summary: in this Mishnah we learned that one who leaves Jerusalem and remembers that he is carrying sacrificial meat - if he has passed Tzofim he burns it where he is, and if not he returns and burns it before the birah with the wood of the arrangement. As to the measure that requires returning, they disagreed: Rabbi Meir - the size of an egg for both, Rabbi Yehudah - a kezayis for both, and the Sages - a kezayis for sacrificial meat and the size of an egg for chametz, because of the stringency of the meat of sacrifices.