Challah, Chapter 4, Mishnah 10. In Maseches Sheviis we learned that Terumah is not brought from outside the Land into Eretz Yisrael, and the same halachah applies to challah. Our Mishnah brings three incidents in which gifts were brought improperly, and they were not accepted.
The first case - Nittai of Tekoa:
"Nittai ish Tekoa heivi challos mi'Beitar, v'lo kiblu mimenu" - Nittai, a man from the city of Tekoa, brought loaves of challah from Beitar. This does not refer to the Beitar in Eretz Yisrael, but to a city by that name outside the Land. For that reason his challah was not accepted from him, since challah is not brought from outside the Land into Eretz Yisrael.
The second case - the people of Alexandria:
"Anshei Alexandria heiviu challosam mei'Alexandria, v'lo kiblu meihem" - the people of Alexandria in Egypt brought their challah from Alexandria, and it was not accepted from them either.
What was to be done with challah that had been brought from outside the Land?
To eat it: impossible, since it had contracted the tumah of the lands of the nations, the tumah that exists outside Eretz Yisrael.
To burn it: impossible, since the tumah of the lands of the nations is not commonly known, and there was a concern that onlookers would say that pure challah was being burned here.
To send it back outside the Land: impossible, since people would say: we saw Terumah leaving Eretz Yisrael for outside the Land, something that is not permitted.
Therefore they would leave it until the eve of Pesach, and burn it together with the chametz.
Bikkurim brought before Shavuos:
"Anshei Har Tzevoim heiviu bikkureihem kodem la'Atzeres, v'lo kiblu meihem" - another matter of this sort, which does not concern challah but rather people who brought a gift not at its proper time: the people of Har Tzevoim brought their bikkurim, that is, their first fruits, before the festival of Shavuos, and they were not accepted from them.
What is the reason? The Torah says: "And the festival of the harvest, the first fruits of your labor, which you sow in the field." The verse is speaking of the festival of Shavuos, which is the festival of the harvest, and which is "the first fruits of your labor" - the first of the fruits of your toil that you sow in the field. Shavuos is the earliest time at which the new fruits are brought; bikkurim are offered from Shavuos onward, but not before it, and therefore they were not accepted from them.
In summary: in this Mishnah we learned of three incidents in which the gifts were not accepted: challah that Nittai of Tekoa brought from Beitar outside the Land, challah that the people of Alexandria brought from Egypt - and since it could not be eaten, burned, or sent back, they would leave it until the eve of Pesach and burn it with the chametz; and bikkurim that the people of Har Tzevoim brought before Shavuos, not at their proper time.