Peah, Chapter Five, Mishnah 8. This mishnah deals with the mitzvah of shichechah (the forgotten sheaf), whose source is the verse: "Ki siktzor ketzircha b'sadecha v'shachachta omer basadeh" - "When you reap your harvest in your field and you forget a sheaf in the field." The Yerushalmi derives a lesson from the parallel between the reaping and the sheaf: just as there is no reaping after the reaping - once the grain has been cut, it is cut - so too there is no sheaving after the sheaving, and the law of shichechah applies only to a gathering that has no further gathering after it.
Shichechah depends on the completion of the work:
It was the practice of the reapers to collect bundles and combine them together, but such a combining is not necessarily the completion of the work. As long as additional bundles are still going to be joined in, this is not the final stage at which all the bundles are gathered together and made ready for the threshing of the stalks into kernels. Wherever there is no completion of the work, there is no law of shichechah, and shichechah applies only at the final place where all the bundles are brought together.
The cases in the mishnah:
"Lekova'os" - small piles that they would make in the field, which look like hats.
"Ulchumsemos" - lower piles, not as tall as the kova'os, resembling small domes.
"Lachararah" - a small pile shaped like a cake.
"Ul'omarim" - small bundles, which will eventually be gathered into larger bundles.
In all of these, "ein lo shichechah" - the law of shichechah does not apply to them, since this is not the completion of the work and their gathering has not yet been finished. In the end, all the kova'os, the domes and the chararos are collected into a larger grouping. But "mimenu ul'goren - yesh lo shichechah": when moving them from the small piles to the threshing floor, the place of threshing, the law of shichechah does apply, because that is the place where the work is completed.
"Hame'amer lagadish" - the gadish is the place where they would bind all the bundles together, even though they did not necessarily thresh there; this is the stage of final binding, and therefore yesh lo shichechah, and one who forgets a bundle there - it is shichechah. But one who takes the entire pile and brings it to the threshing floor, where the actual threshing is done - ein lo shichechah, since the binding has already been completed.
This is the rule: whenever one gathers into a place that is the completion of the work, meaning the place where all the bundles are brought together - there is shichechah at that stage. But once he has reached the completion of the work and all the bundles are bound, and all that remains is to move them from place to place for the threshing - there is no longer any concept of shichechah. And whenever one gathers into a place that is not the completion of the work, such as those small piles - there is no shichechah there, but when they are moved from there to the threshing floor, which is the final stage of gathering, the mitzvah of shichechah does apply.
In summary: the law of shichechah depends entirely on the question of whether the place to which one gathers is a place where the work is completed. Gathering into kova'os, chumsemos, a chararah or omarim is not the completion of the work and there is no shichechah in it, but moving them to the threshing floor - there is shichechah. Gathering into a gadish, which is the final binding - there is shichechah in it, and moving it from there to the threshing floor - there is no shichechah.