בכורות א׳Bekhorot 1

Mishnah 1א׳

הַלּוֹקֵחַ עֻבַּר חֲמוֹרוֹ שֶׁל נָכְרִי, וְהַמּוֹכֵר לוֹ אַף עַל פִּי שֶׁאֵינוֹ רַשַּׁאי, וְהַמִּשְׁתַּתֵּף לוֹ, וְהַמְקַבֵּל מִמֶּנּוּ, וְהַנּוֹתֵן לוֹ בְקַבָּלָה, פָּטוּר מִן הַבְּכוֹרָה, שֶׁנֶּאֱמַר (במדבר ג), בְּיִשְׂרָאֵל, אֲבָל לֹא בַאֲחֵרִים. כֹּהֲנִים וּלְוִיִּם פְּטוּרִין מִקַּל וָחֹמֶר, אִם פָּטְרוּ שֶׁל יִשְׂרָאֵל בַּמִּדְבָּר, דִּין הוּא שֶׁיִּפְטְרוּ שֶׁל עַצְמָן:

If one buys an unborn donkey fetus from a non-Jew, sells [an unborn donkey fetus] to [a non-Jew] even though one does not have permission to, has a partnership with [a non-Jew], receives [an unborn donkey fetus] from [a non-Jew], or gives [an unborn donkey fetus] to [a non-Jew], [the unborn donkey fetus] is exempt from the laws of the first-born. As it is written (Numbers 3:13): "in Israel", but not by others. Priests and Levites are also exempt, for if they could exempt a Israelite [while the Jewish people were] in the desert, then they would certainly exempt themselves.

Mishnah 2ב׳

פָּרָה שֶׁיָּלְדָה כְּמִין חֲמוֹר, וַחֲמוֹר שֶׁיָּלְדָה כְּמִין סוּס, פָּטוּר מִן הַבְּכוֹרָה, שֶׁנֶּאֱמַר פֶּטֶר חֲמוֹר פֶּטֶר חֲמוֹר, שְׁנֵי פְעָמִים, עַד שֶׁיְּהֵא הַיּוֹלֵד חֲמוֹר וְהַנּוֹלָד חֲמוֹר. וּמָה הֵם בַּאֲכִילָה. בְּהֵמָה טְהוֹרָה שֶׁיָּלְדָה כְּמִין בְּהֵמָה טְמֵאָה, מֻתָּר בַּאֲכִילָה. וּטְמֵאָה שֶׁיָּלְדָה כְּמִין בְּהֵמָה טְהוֹרָה, אָסוּר בַּאֲכִילָה, שֶׁהַיּוֹצֵא מֵהַטָּמֵא, טָמֵא. וְהַיּוֹצֵא מִן הַטָּהוֹר, טָהוֹר. דָּג טָמֵא שֶׁבָּלַע דָּג טָהוֹר, מֻתָּר בַּאֲכִילָה. וְטָהוֹר שֶׁבָּלַע דָּג טָמֵא, אָסוּר בַּאֲכִילָה, לְפִי שֶׁאֵינוֹ גִדּוּלָיו:

If a cow gives birth to [a creature which] looks like a donkey or a donkey gives birth to [a creature which] looks like a horse, it is exempt from the laws of the firstborn, for it is written (Exodus 34:20": "firstborn donkey" and (Exodus 13:13) "firstborn donkey"--two times, [to teach that] the birthing [mother] must be a donkey, and the born [creature] must be a donkey. And what about eating them? If a pure species gives birth to [a creature which] looks like an impure species, it is permissible to eat [the offspring]; if an impure species gives birth to [a creature which] looks like a pure species, it is forbidden to eat [the offspring], for that which comes from an impure species is impure and that which comes from a pure species is pure. If an impure fish swallows a pure fish, [the pure fish found inside] is permissible to eat; if a pure fish swallows an impure fish, [the impure fish found inside] is forbidden to eat, for it is not its product.