Maseches Beitzah, Chapter 1, Mishnah 6. This mishnah deals with two of the gifts given to the kohanim, and with the question of whether or not they may be brought to a kohen on Yom Tov.
The two gifts discussed in the mishnah:
Challah - a piece of dough taken from any large quantity of dough that a person prepares, and given to a kohen.
Matanos (gifts) - although the word itself is general, the reference here is to the foreleg, the jaw and the maw: three portions of an animal that one who slaughters an animal must give to a kohen. These gifts are eaten by any kohen and by the members of his family, and they need not be eaten in a state of purity - even a person who is tamei may eat them. The foreleg is the right front leg, the jaw is the lower part of the jawbone, and the maw is part of the stomach.
The position of Beis Shammai:
"Beis Shammai omrim, ein molichin challah u'matanos la'kohen b'Yom Tov" - one may not bring the challah and the gifts to a kohen on Yom Tov, and one may not give them to him on that day. This applies whether they were separated before Yom Tov or on Yom Tov itself.
Why would they be separated on Yom Tov at all? Because on Yom Tov, unlike Shabbos, one is permitted to knead dough and one is permitted to slaughter an animal, and therefore the obligation in these two gifts can arise, and they can be separated, on Yom Tov itself. Even so, Beis Shammai rule that they are not brought to a kohen on Yom Tov, and the reason will be explained shortly. "U'Beis Hillel matirin" - and Beis Hillel permit it.
The argument of Beis Shammai - a "gezeirah shavah":
Ordinarily a gezeirah shavah is one of the thirteen principles by which the Torah is expounded: a derivation from two identical or similar words, which creates a bridge between two separate areas of Torah, and through it laws are learned from one to the other. Here Beis Shammai borrow the term, even though this is not truly a gezeirah shavah - after all, the matters under discussion are rabbinic only - but rather a comparison between two laws.
And this is what Beis Shammai say: "Challah u'matanos matanah la'kohen, u'Terumah matanah la'kohen; k'shem she'ein molichin es ha'Terumah - kach ein molichin es ha'matanos" - challah and the gifts are a gift to the kohen, and Terumah is a gift to the kohen; just as one does not bring the Terumah, so too one does not bring the gifts.
The reason one does not bring Terumah to a kohen on Yom Tov is that there is no way to separate terumos and maasros on Yom Tov: the actions that create the obligation in Terumah involve gathering the pile of grain and smoothing it out, and these actions are forbidden on Yom Tov. It emerges that a person cannot become obligated in Terumah on Yom Tov, and so he should have separated it and given it beforehand. On the strength of this comparison, Beis Shammai rule sweepingly that no gifts of the kehunah are brought to a kohen on Yom Tov.
The response of Beis Hillel:
Beis Hillel disagree and say: "Lo" - no, the two cases are not the same. Regarding Terumah this law was stated because a person has no entitlement to separate it on Yom Tov: he simply cannot arrive at a situation in which he would separate Terumah on Yom Tov. Since the obligation already came into being before Yom Tov, he should have separated the Terumah before the festival, and therefore he is not permitted to bring it on Yom Tov.
But regarding the gifts - in which he is entitled to separate them - the law is different. A person can come to be obligated to separate them on Yom Tov itself: he may knead dough on Yom Tov and become obligated in challah, and he may slaughter an animal on Yom Tov and become obligated in the foreleg, the jaw and the maw. Since the obligation arises anew on Yom Tov, these gifts are essentially different from Terumah, and therefore they may be given to a kohen on Yom Tov.
The conclusion of the Gemara:
In practice, the Gemara reaches a different conclusion regarding our mishnah: Beis Shammai and Beis Hillel both agree that challah and the gifts may be brought to a kohen on Yom Tov, and their entire dispute concerns Terumah that was separated before Yom Tov - whether it may be brought to a kohen on Yom Tov. That, according to the Gemara, is the point of dispute.
In summary: in this mishnah we dealt with two of the gifts of the kehunah - challah, and the foreleg, jaw and maw. Beis Shammai forbid bringing them to a kohen on Yom Tov based on the comparison to Terumah, while Beis Hillel permit it, because with these gifts the entitlement and the obligation can arise on Yom Tov itself. The Gemara understands the dispute differently: all agree that challah and the gifts may be brought on Yom Tov, and the dispute concerns Terumah that was separated while it was still day.