Sukkah Chapter 3, Mishnah 8. The central issue debated by the Tannaim in this Mishnah revolves around a single question: is there an obligation to bind the lulav together with the other species, or not? This question has a practical halachic consequence, as we will see in the wording of the Mishnah.
The View of Rabbi Yehudah:
"Ein ogdin es halulav ela b'mino" - one may bind the lulav only with something that belongs to the same species as the lulav and the other species. In practice the custom is to bind it with the leaves of the lulav itself, from which rings and strips are fashioned to tie the species together; but one may not bind it with a different material, such as a gold band. This is stated here in the name of Rabbi Yehudah.
Rabbi Yehudah's Reasoning:
Rabbi Yehudah holds that there is an obligation to bind the lulav, and therefore the binding itself is part of the mitzvah. Consequently, one who binds it with something that is not one of the four species thereby adds a fifth species, and may transgress "bal tosif" (the prohibition against adding to a mitzvah). The Torah commanded us to take four species - four and not five; and one who brings an additional type, according to Rabbi Yehudah who holds that the binding is a full obligation, is adding to the mitzvah.
The View of Rabbi Meir:
"Rabbi Meir omer: afilu b'meshichah" - in his opinion one may bind the lulav even with a cord, and even with a different material, because he holds that "a lulav does not require binding": there is no absolute obligation for the species to be tied together. The binding is merely an accessory, and is not considered part of the act of the mitzvah itself.
Rabbi Meir's Proof from an Incident:
Rabbi Meir seeks to prove his position from an incident that occurred: "Ma'aseh b'anshei Yerushalayim shehayu ogdin es lulaveihen b'gimoniyos shel zahav" - the people of Jerusalem would tie their lulavim with gold threads, which shows that one may bind it even with another material.
The Sages' Response:
"Amru lo: b'mino hayu ogdin oso milmatah" - there is no proof from that incident, since in fact they would bind the lulav below with its own species, with the rings of the lulav, and with that binding they fulfilled the mitzvah. There was therefore no added species here. The gold above was not part of the mitzvah at all, and they did not intend to fulfill the mitzvah with it, but only for beauty - and beautifying is permitted. It emerges that the Sages disagreed with Rabbi Meir about the very facts of the incident.
In summary: In this Mishnah the Tannaim disagree about whether a lulav requires binding. According to Rabbi Yehudah the binding is part of the mitzvah, and therefore one may bind it only with its own species, for otherwise he adds a fifth species and transgresses "bal tosif"; and according to Rabbi Meir the lulav does not require binding, and the binding is merely an accessory, and therefore it may be bound even with a cord. Rabbi Meir brought a proof from the practice of the people of Jerusalem, who bound their lulavim with gold threads, and the Sages rejected his proof: they would bind it below with its own species, and the gold was there for beauty alone.