Bikkurim, Chapter 3, Mishnah 11, which deals with the law of the addition made to the bikkurim.
The text of the Mishnah:
"Eimasai amru tosefes habikkurim k'bikkurim? Bizman shehi ba'ah min ha'aretz; v'im einah ba'ah min ha'aretz, einah k'bikkurim"
Explanation:
"Eimasai amru tosefes habikkurim k'bikkurim" - When do we say that the addition made to the bikkurim has the same status as the bikkurim themselves?
"Bizman shehi ba'ah min ha'aretz" - When the addition is brought from the Land of Israel, from produce that grew in the Land.
"V'im einah ba'ah min ha'aretz, einah k'bikkurim" - But if one makes the addition to the bikkurim from produce that comes from outside the Land of Israel, it does not have the status of bikkurim.
The words of the Rambam:
The Rambam writes that even though this addition does not have the status of bikkurim, since the person designated it as an addition to the bikkurim, it must be eaten in purity. Nevertheless, it is not truly bikkurim, because the addition to the bikkurim must come from the Land of Israel itself.
In summary: The addition to the bikkurim is treated as bikkurim only when it comes from the Land of Israel; produce brought from outside the Land is not treated as bikkurim, and according to the Rambam only this much applies - that because of the name of "addition" given to it, it is eaten in purity.