Rosh Hashanah, Chapter 4, Mishnah 8. This Mishnah teaches us that one may not desecrate Rosh Hashanah and Yom Tov, even by a rabbinic prohibition alone, in order to fulfill the mitzvah of shofar - even though hearing the shofar is a mitzvah from the Torah.
"Shofar shel Rosh Hashanah, ein ma'avirin alav es hatechum":
A person may not go beyond the Shabbos boundary in order to bring a shofar or in order to hear the blasts. Although hearing the shofar is a positive commandment, Yom Tov itself includes both a positive and a negative commandment, and a positive commandment does not override a positive plus a negative commandment. More than that: even when only a rabbinic prohibition is involved, we do not set it aside for the sake of the positive commandment of shofar.
The Mishnah continues and lists further cases:
"Ein mefakchin alav es hagal" - if a pile of stones fell on the shofar, one does not clear away the stones, which are muktzeh, in order to blow it.
"Lo olin ba'ilan" - climbing a tree is forbidden by rabbinic decree, lest one detach a branch.
"Lo rochvin al gabei behemah" - riding an animal in order to obtain a shofar is likewise forbidden by rabbinic decree, lest one detach a branch in order to drive the animal properly.
"Lo shatin al penei hamayim" - swimming is forbidden by rabbinic decree, lest one fashion a flotation device.
"Ve'ein choschin oso":
If the shofar needs to be trimmed, one does not cut it - "bein bedavar shehu mishum shevus", with an implement forbidden by rabbinic decree, "uvein bedavar shehu mishum lo sa'aseh", in the ordinary manner of cutting, which is forbidden from the Torah. Both ways are forbidden.
"Aval im ratzah litein lesocho mayim o yayin - yitein":
One may soak the shofar in water or in wine so that its sound will be more pleasant. This is permitted because it involves no repair at all, only sweetening the tone.
"Ein me'akvin es hatinokos miltko'a":
We do not prevent children from blowing the shofar on Rosh Hashanah, "aval mis'askin imahem ad sheyilmedu" - on the contrary, we occupy ourselves with them until they learn how to blow. The Gemara says that this is true even on Shabbos: although we do not blow the shofar on Shabbos, children are permitted to do so on a Rosh Hashanah that falls on Shabbos.
The law of mis'asek:
Since we have been dealing with the idea of blowing merely for the sake of practice, as mentioned regarding the children, the rule must be made clear: one who blows as a mis'asek has not fulfilled the mitzvah.
The one who blows: an adult who blows the shofar solely for practice, without intent to fulfill the mitzvah, has not fulfilled his obligation.
The one who hears: likewise, one who hears a blast from a person who blew as a mis'asek, for practice and not for the sake of the mitzvah, has not fulfilled his obligation. The blowing must be done for the sake of the mitzvah, and not merely to practice producing the correct sounds.
In summary: in this Mishnah we learned that the prohibitions of Yom Tov are not set aside for the mitzvah of shofar, not even rabbinic prohibitions - not going beyond the Shabbos boundary, not clearing a pile of stones, not climbing a tree, not riding an animal and not swimming, and likewise one does not cut the shofar, neither through a rabbinic prohibition nor through a Torah prohibition. On the other hand, one may place water or wine into it, since this involves no repair. We also learned that we do not prevent children from blowing but rather occupy ourselves with them until they learn, and even so - the blast of a mis'asek does not fulfill the obligation, neither for the one who blows nor for the one who hears.