Yoreh Deah Siman 25 Seif 2

ב הַבּוֹדְקִים בְּנוֹצָה לִרְאוֹת אִם נִשְׁחַט הָרֹב, טוֹעִים הֵם.

 

Seif 2

 

Those who do the checking with a feather to see if the slaughter was done complete/proper/ to a sizable amount, they are making an error.