Yoreh Deah Siman 18 Seif 5

 

ה סַכִּין שֶׁיֵּשׁ בָּהּ פְּגִימוֹת הַרְבֵּה, אֲפִלּוּ כֻּלָּן מְסֻכְסָכוֹת, אֲפִלּוּ שָׁחַט בָּהּ בְּדִיעֲבַד, פְּסוּלָה. דְּמִתּוֹךְ שֶׁפְּגִימוֹתֶיהָ מְרֻבּוֹת חַיְשִׁינָן שֶׁמָּא הֵשִׁיב יָדוֹ מְעַט וְלֹא הִרְגִּישׁ

 

Seif 5

A knife that has many flaws, even if everyone disputes them, even if slaughtered (under the rubric of) after the fact, it is not acceptable. Because the flaws are many we worry when he returns his hand and kills (he will) not feel it.