בכורות ה׳Bekhorot 5

Mishnah 3ג׳

הַצּוֹרֵם בְּאֹזֶן הַבְּכוֹר, הֲרֵי זֶה לֹא יִשָּׁחֵט עוֹלָמִית, דִּבְרֵי רַבִּי אֱלִיעֶזֶר. וַחֲכָמִים אוֹמְרִים, כְּשֶׁיִּוָּלֵד לוֹ מוּם אַחֵר, יִשָּׁחֵט עָלָיו. מַעֲשֶׂה בְזָכָר שֶׁל רְחֵלִים זָקֵן וּשְׂעָרוֹ מְדֻלְדָּל, רָאָהוּ קַסְדּוֹר אֶחָד, אָמַר, מַה טִּיבוֹ שֶׁל זֶה. אָמְרוּ לוֹ, בְּכוֹר הוּא וְאֵינוֹ נִשְׁחָט אֶלָּא אִם כֵּן הָיָה בּוֹ מוּם. נָטַל פִּגְיוֹן וְצָרַם בְּאָזְנוֹ, וּבָא מַעֲשֶׂה לִפְנֵי חֲכָמִים וְהִתִּירוּהוּ. רָאָה שֶׁהִתִּירוּ, וְהָלַךְ וְצָרַם בְּאָזְנֵי בְכוֹרוֹת אֲחֵרִים, וְאָסָרוּ. פַּעַם אַחַת הָיוּ תִינוֹקוֹת מְשַׂחֲקִין בַּשָּׂדֶה וְקָשְׁרוּ זַנְבוֹת טְלָאִים זֶה לָזֶה, וְנִפְסְקָה זְנָבוֹ שֶׁל אֶחָד מֵהֶם וַהֲרֵי הוּא בְכוֹר, וּבָא מַעֲשֶׂה לִפְנֵי חֲכָמִים וְהִתִּירוּהוּ. רָאוּ שֶׁהִתִּירוּ, וְהָלְכוּ וְקָשְׁרוּ זַנְבוֹת בְּכוֹרוֹת אֲחֵרִים, וְאָסָרוּ. זֶה הַכְּלָל, כֹּל שֶׁהוּא לְדַעְתּוֹ, אָסוּר. וְשֶׁלֹּא לְדַעְתּוֹ, מֻתָּר:

If one slits the ear of a firstborn it may never be slaughtered, according to Rabbi Eli'ezer. The sages say: Once it develops another blemish it may be slaughtered on that [basis]. It once happened that a Roman official saw an old ram with its wool matted and asked: "What is with this one?" They answered: "It is a firstborn and it may not be slaughtered unless it develops a blemish." He took a dagger and slit its ear. It came before the sages, and they permitted it. Once he saw that [the sages] permitted it, he went and made slits in the ears of other firstborns, and [the sages] forbade them. It once happened that children were playing in a field, and they tied the tails of two lambs to each other. One of the tails, belonging to a firstborn, was detached. This case came before the sages and they permitted it. Once [the people] saw that [the sages] permitted it, [the people] went and tied the tails of other firstborns together, but [the sages] forbade them. This is the rule, anything intentional is forbidden, and that which is unintentional is permitted.

Mishnah 4ד׳

הָיָה בְכוֹר רוֹדְפוֹ, וּבְעָטוֹ וְעָשָׂה בוֹ מוּם, הֲרֵי זֶה יִשְׁחוֹט עָלָיו. כָּל הַמּוּמִין הָרְאוּיִין לָבוֹא בִידֵי אָדָם, רוֹעִים יִשְׂרָאֵל נֶאֱמָנִים, וְרוֹעִים כֹּהֲנִים אֵינָן נֶאֱמָנִים. רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל אוֹמֵר, נֶאֱמָן הוּא עַל שֶׁל חֲבֵרוֹ וְאֵינוֹ נֶאֱמָן עַל שֶׁל עַצְמוֹ. רַבִּי מֵאִיר אוֹמֵר, הֶחָשׁוּד עַל דָּבָר, לֹא דָנוֹ וְלֹא מְעִידוֹ:

If one is chased by a firstborn, and one kicks it and causes a blemish, one may slaughter it on that [basis]. Israelite shepherds are trusted [regarding] all blemishes which could come about by human [causation] but priests [who are] shepherds are not trusted. Rabban Shim'on ben Gamliel says: [The priest] is trusted regarding [the firstborn of] a friend but is not trusted regarding [the priest's] own. Rabbi Meir says: One who is suspected regarding something cannot judge [cases regarding] it and cannot testify [about] it.