Yoreh Deah Siman 18 Seif 11

 

יא הַשּׁוֹחֵט בְּהֵמוֹת רַבּוֹת אוֹ עוֹפוֹת הַרְבֵּה, צָרִיךְ לִבְדֹּק בֵּין כָּל אֶחָד וְאֶחָד. שֶׁאִם לֹא עָשָׂה כֵּן וּבָדַק בָּאַחֲרוֹנָה, וְנִמְצֵאת סַכִּין פְּגוּמָה; הֲרֵי הַכֹּל סְפֵק נְבֵלוֹת וַאֲפִלּוּ הָרִאשׁוֹנָה.

Seif 11

 

If a person slaughters many animals or many fowl, he must inspect [the knife] between each [slaughter]. If he did not check, and then checked after slaughtering the last one and discovered [the knife] to be blemished, there is an unresolved doubt whether all of them even the first are considered nevelot.