Pe'ah, Chapter 6, Mishnah 2. This mishnah discusses the dispute between Beis Shammai and Beis Hillel as to what is considered shichechah (a forgotten sheaf).
The Content of the Mishnah:
A sheaf of grain that is lying next to one of four distinct landmarks, and the owner of the field forgot it:
"Gefah" - a stone fence, which is not cemented together with mortar but rather stones placed one on top of another.
"Gadish" - a large pile of sheaves of grain.
"Bakar" - the place where the animals are kept.
"Kelim" - the place where the plowing implements are kept.
The Dispute Between Beis Shammai and Beis Hillel:
Beis Shammai: This is not shichechah, since those items next to which the sheaf is lying serve as a marker for it, and even if the owner does not remember it at present, he will eventually remember it.
Beis Hillel: This is shichechah. The marker alongside it is not enough to cancel the law of shichechah, and once he forgot it, he forgot it, and it belongs to the poor.
The Approach of Rabbi Eliezer in the Yerushalmi:
The Yerushalmi teaches that this version of the dispute follows the understanding of Rabbi Yehoshua, whereas Rabbi Eliezer understands the dispute between Beis Shammai and Beis Hillel differently.
According to Rabbi Eliezer, one must distinguish between two cases:
A sheaf simply lying there: A sheaf that is lying beside one of these items - a gefah, a gadish, the animals or the implements - and he forgot it, everyone agrees, both Beis Shammai and Beis Hillel, that this is shichechah. The mere fact that it is lying next to a marker does not prevent it from being forgotten, and it belongs to the poor.
A sheaf that was lifted up: What, then, is their dispute about? About a sheaf that a person lifted up in order to bring it to the city or to his home, and he set it down beside that gefah or gadish or the animals or the implements, and afterward forgot it. Beis Shammai say that this is not shichechah, since he already acquired it when he lifted it up; and Beis Hillel say that it is still shichechah, since afterward he forgot it.
In Summary: In this mishnah we learned that Beis Shammai and Beis Hillel disagree about a sheaf lying beside a prominent marker - a gefah, a gadish, the animals or the implements. According to Rabbi Yehoshua, their dispute is about a sheaf that is simply lying there: Beis Shammai hold that this is not shichechah because of the marker, and Beis Hillel hold that it is shichechah. According to Rabbi Eliezer, in the case of a sheaf simply lying there everyone agrees that it is shichechah, and they disagree only about a sheaf that was lifted up in order to be brought home and was set down there and forgotten, where Beis Shammai hold that he has already acquired it, while Beis Hillel hold that the law of shichechah still applies to it.