Bikkurim, Chapter 2, Mishnah 5. This mishnah deals with terumas maaser - the ten percent taken from the maaser held by the Levi and given to the kohen - and with the question of in what way it resembles bikkurim and in what way it resembles Terumah. In the words of the mishnah: terumas maaser is like bikkurim in two ways, and like Terumah in two ways.
The two ways in which it resembles bikkurim:
Let us first introduce the law of Terumah: one does not separate Terumah unless it is min hamukaf, adjacent to the produce. The Terumah that a person separates, designates and gives to the kohen must be next to the tevel from which he is separating it. And since the two must be found side by side, one does not separate from tahor produce on behalf of tamei produce, lest their proximity to one another cause the potential Terumah to become tamei. It turns out that these two laws are really two sides of a single idea.
Regarding terumas maaser our mishnah teaches: "nittelet min hatahor al hatamei v'shelo min hamukaf k'vikkurim" - it is taken from tahor produce on behalf of tamei produce and not from adjacent produce, like bikkurim:
"shelo min hamukaf" - the terumas maaser may be in one place while the maaser rishon from which it is being separated is far away from it, and the two need not be adjacent. And the same is the law regarding bikkurim.
"min hatahor al hatamei" - since they need not be adjacent, there is no concern at all in separating from tahor produce on behalf of tamei produce. And the same is the law regarding bikkurim.
The two ways in which it resembles Terumah:
"v'oseret et hagoren" - as long as the terumas maaser has not been separated, the goren, meaning the pile of grain, is forbidden to be eaten until he separates it. With bikkurim, as we learned in the previous mishnah, this is not the case: they do not forbid the goren, and the grain is not considered tevel because he has not separated bikkurim.
"v'yeish lah shiur k'terumah" - terumas maaser has a fixed measure, one in ten, and Terumah too has a measure.
A precise point is in order regarding the measure: although we mentioned in the previous mishnah that bikkurim were also given a measure, that measure is not hinted at anywhere in the Torah. The measure of Terumah, by contrast, even though it is rabbinic, is hinted at in the Torah as an asmachta. On this point the two differ from one another: terumas maaser and Terumah have a source in the Torah for their measure, and bikkurim do not.
In summary: in this mishnah we learned that terumas maaser stands between the two mitzvos: like bikkurim it is taken from tahor produce on behalf of tamei produce and not from adjacent produce, and like Terumah it forbids the goren until it is separated and it has a fixed measure that is hinted at in the Torah.