Pesachim, Chapter 3, Mishnah 6. This mishnah deals with a case in which the fourteenth of Nissan, erev Pesach, falls on Shabbos. The question it addresses is how much of the chametz must be destroyed before Shabbos begins, and how much may be left over to be destroyed on Shabbos itself.
Three opinions are stated in the mishnah:
Rabbi Meir: "Arba'ah asar shechal lihyos b'Shabbos - me'va'arin es hakol milifnei haShabbos, divrei Rabbi Meir" - in his view, all the chametz must be destroyed before Shabbos begins, both chullin and Terumah, except for the amount needed for eating on Shabbos itself.
The Sages: "V'chachamim omrim: bizmanan" - there is no need to destroy everything ahead of time, before Shabbos. Rather, the chametz may be destroyed at its proper time, on Shabbos: by eating it or feeding it to animals, and in this way all of it will be finished before the end of the time for eating chametz. There is therefore no reason for concern or worry before Shabbos.
Rabbi Elazar bar Tzadok: "Terumah milifnei haShabbos v'chullin bizmanan" - he distinguishes between the two types of food. Terumah must be destroyed before Shabbos, since only a few people are permitted to eat it - only a kohen, and in a state of purity - and there are not enough eaters to finish it in time. With chullin, however, one may rely on the words of the Sages, since there are enough eaters to finish it in the course of Shabbos: the person himself, his animals and the like, and he is able to dispose of it before the end of the time for eating chametz.
In summary: in this mishnah the Tannaim disagree about the law when erev Pesach falls on Shabbos. Rabbi Meir requires that everything be destroyed before Shabbos, apart from the food for Shabbos; the Sages permit destroying it all at its proper time on Shabbos; and Rabbi Elazar bar Tzadok distinguishes between Terumah, for which there are not enough eaters to finish it and which is therefore destroyed before Shabbos, and chullin, which is destroyed at its proper time.