סימן כז – שלא לחתך אבר מהבהמה בעודה מפרכסת, ובו סעיף אחד
Siman 27 That one may not cut a limb from an animal while it is still in its mefarcheset (post death muscle movement)
א חָתַךְ מִבְּהֵמָה לְאַחַר שֶׁנִּשְׁחֲטָה כָּרָאוּי וְעוֹדֵּנָהּ מְפַרְכֶּסֶת, אָסוּר לֶאֱכֹל מִמֶּנָּהּ עַד שֶׁתָּמוּת הַבְּהֵמָה.
Seif 1
If one cuts from an animal a piece of meat after it was slaughtered properly and it is still twitching it is forbidden to eat from it before it (the animal) dies.