יב

הַשּׁוֹחֵט בִּפְנֵי עֵדִים בְּהֵמָה לְעוֹבֵד כּוֹכָבִים, וּכְשֶׁבָּא יִשְׂרָאֵל לִקְנוֹת מִמֶּנָּה, אָמַר: לֹא תִּקְנֶה מִמֶּנָּה כִּי לֹא שְׁחַטְתִּיהָ, אֵינוֹ נֶאֱמָן. וּמִיהוּ לְדִידֵיהּ אֲסוּרָה, דְּהָא שָׁוְיָא אַנַּפְשֵׁיהּ חֲתִיכָה דְּאִסוּרָא.

 

Seif 12

 

If someone slaughters an animal for a non-Jew in the presence of witnesses, and a Jew comes to purchase it from him, and he says "do not buy it, because it wasn't slaughtered in a kosher manner", we do not believe him, [and the buyer is allowed to purchase it (*based on the testimony of the witnesses who saw it was slaughtered in a  kosher manner)].  However, for the shochet himself, it is forbidden for him to eat it, since he declared it to be a forbidden item for himself (i.e. by his statement that it wasn't kosher).