Yoreh Deah Siman 17 Seif 2

ב הַשּׁוֹחֵט אֶת הַמְסֻכֶּנֶת בַּלַּיְלָה וְלֹא יָדַע אִם פִּרְכְּסָה, הֲרֵי זוֹ סְפֵק נְבֵלָה וַאֲסוּרָה.

 

Seif 2

When one slaughters an animal that is dangerously ill at night and does not know whether or not it made convulsive movements, it is forbidden, because of the possibility that it is forbidden as nevelah.