Yoreh Deah Siman 17 Seif 3

ג גְּדוֹלֵי הַחֲכָמִים לֹא הָיוּ אוֹכְלִים מִבְּהֵמָה שֶׁמְּמַהֲרִים וְשׁוֹחֲטִים אוֹתָהּ כְּדֵי שֶׁלֹּא תָּמוּת, וְאַף עַל פִּי שֶׁפִּרְכְּסָה בְּסוֹף הַשְּׁחִיטָה; וְדָבָר זֶה אֵין בּוֹ אִסוּר, אֶלָּא כָּל הָרוֹצֶה לְהַחְמִיר עַל עַצְמוֹ בְּדָבָר זֶה הֲרֵי זֶה מְשֻׁבָּח.

Seif 3

Our great sages would not partake would not eat of an animal which people were hurrying to slaughter before it died, even if it made convulsive movements after being slaughtered. This is an issue that does not involve a prohibited act. Nevertheless, if one desires to accept this stringency upon himself it is considered praiseworthy.