Beitzah, Chapter Five, Mishnah 6. The Mishnah deals with a person whose produce is located in another city, outside the boundary within which he is permitted to walk, while the residents of that city placed an eruv techumin midway along the road between the two cities, thereby enabling themselves to come to him. The question is whether they may also bring his produce along with them.
The words of the Mishnah:
"Mi shehayu peirosav be'ir acheres" - a person whose produce is located in another city, outside his boundary.
"Ve'ervu benei osah ha'ir lehavi etzlo mipeirosav" - people from the city where the produce is located placed an eruv techumin so that they would be able to walk to the other city, and they wish to bring some of his produce with them.
"Lo yavi'u lo" - they are not permitted to do so.
"Ve'im eirev hu - peirosav kamohu" - if the owner himself placed an eruv, the produce is like him: just as he is permitted to travel from city to city, so his produce is permitted to travel with him.
The reason for this: the eruv made by the residents of that city works only for themselves, and it has no power to affect the produce, since the produce does not belong to them but to its owner, and he did not place an eruv techumin connecting the two cities. Therefore, even though they may come, the produce must remain in its place.
The Gemara's clarification:
The Gemara clarifies that the ruling of the Mishnah, that the produce follows the boundary of its owner, was stated specifically in a particular case, and one must distinguish between two situations:
He designated a place for it: when he deposited his produce with the people of that city and designated a special place for it, the ones with whom it was deposited did not accept responsibility for guarding the produce, since the owner guarded it himself by placing it in a special place. Therefore the produce has the boundary status of the owner, and it cannot be brought to him because it is outside his boundary - unless he placed an eruv.
He did not designate a place for it: when he deposited the produce and did not set aside a separate place for it, but rather left it in the hands of the one with whom he deposited it, then even though the produce belongs to him financially, since it is under the control and responsibility of the one holding it, it takes on the boundary status of that person. In this case the ruling of the Mishnah does not apply, and the residents of the city may bring the produce, since the produce follows their own boundary.
In summary: in the Mishnah we learned that produce located in another city follows the boundary of its owner, and therefore the residents of the city who made an eruv may not bring it, and only if the owner himself made an eruv - "peirosav kamohu". The Gemara clarified that this ruling was stated where he designated a place for the produce and the one holding it did not accept responsibility for it; but where he did not designate a place for it and it is entrusted to the responsibility of the one holding it, it follows that person's boundary and it may be brought.