Pe'ah, Chapter 2, Mishnah 6. This mishnah gives us the historical background to the halachah taught in the previous mishnah. There we learned that two kinds of wheat are considered two species when they are harvested at two different times, whereas when they are harvested together at the same time they are one species with regard to pe'ah.
The case brought before Rabban Gamliel:
"Ma'aseh she'zara Rabbi Shimon Ish HaMitzpeh lifnei Rabban Gamliel" - the mishnah means that Rabbi Shimon Ish HaMitzpeh planted his field, and afterward brought his question before Rabban Gamliel. The question concerned two types of wheat that he had planted, and he wanted to know whether they count as one species with regard to pe'ah or as two species.
"Ve'alu l'Lishkas HaGazis v'sha'alu" - the Lishkas HaGazis is the chamber in the Beis HaMikdash where the Great Sanhedrin, the highest court, would sit, and there they presented their question.
The chain of transmission:
"Amar Nachum HaLavlar: mekubal ani m'Rabbi Miyasha" - Nachum HaLavlar, one of the members of the Sanhedrin, testified that he possessed a tradition which he had received and learned from Rabbi Miyasha. And this is how that tradition was passed down:
Rabbi Miyasha - from him Nachum HaLavlar received it.
"She'kibel me'Abba" - Rabbi Miyasha received it from his father.
"She'kibel min ha'zugos" - and his father received it from the Pairs.
"She'kiblu min ha'nevi'im" - and the Pairs received it from the prophets.
The Pairs are the av beis din and the nasi of the courts of the Sanhedrin in each and every generation, beginning with Shammai and Hillel and going back to Shemayah and Avtalyon and so on - as they are all listed at the beginning of Pirkei Avos - all the way to Shimon HaTzaddik and the Men of the Great Assembly. Among the Men of the Great Assembly were prophets: Chaggai, Zechariah and Malachi, and it was from them that the halachah was transmitted.
The halachah given to Moshe at Sinai:
"She'hi halachah l'Moshe mi'Sinai, she'ha'zorea es sadeihu shnei minei chitim" - this is the halachah that was given to Moshe at Sinai regarding a person who plants two different kinds of wheat in his field:
"Im asa'an goren achas - nosen pe'ah achas" - if he harvested it all at one time, he gives one pe'ah from the entire field, since it is considered a single field, even though two sub-species of wheat are involved.
"Shtei goranos - nosen shtei pe'os" - if he harvested them at two different times, they are considered two species and two separate fields, and therefore he gives two pe'os.
In summary: this mishnah relates an incident involving Rabbi Shimon Ish HaMitzpeh, who brought his question before Rabban Gamliel, and from there they went up to the Lishkas HaGazis and asked the Sanhedrin. Nachum HaLavlar testified to a tradition that he had received person from person - from Rabbi Miyasha, from his father, from the Pairs and from the prophets - that it is a halachah given to Moshe at Sinai: one who plants two kinds of wheat in his field, if he harvested them as one harvest he gives one pe'ah, and if he harvested them as two harvests he gives two pe'os.