Yoreh Deah Siman 24 Seif 10

 

י אִם לְאַחַר שֶׁשָּׁחַט רֹב הַסִימָנִים הֶחֱלִיד הַסַכִּין תַּחַת מִעוּט הַנִּשְׁאָר מִשְּׁנֵיהֶם אוֹ מֵאֶחָד מֵהֶם וּפְסָקוֹ, מֻתָּר. וְיֵשׁ מִי שֶׁאוֹסֵר גַּם בָּזֶה, וְרָאוּי לָחוּשׁ לִדְבָרָיו לְכַתְּחִלָּה.

 

Seif 10

 

If after one slaughtered most of the signs and the knife becomes hidden under the remaining minority [of the signs], or from one of them [the signs] and divided [cut] it, it [the slaughter] is permitted. There are those [that say] that this is also forbidden and it is clear that there is concern in the matter ab initio (before the fact).