Reference for Bava Kamma 198:18

ההוא מגרומתא דאתאי לקמיה דרב טרפיה ופטריה לטבח מלשלומי דמי פגעו ביה רב כהנא ורב אסי בההוא גברא אמרו ליה עביד בך רב תרתי

and he said to the slaughterer. 'Go and bring evidence that you are skilled to slaughter hens, and I will declare you exempt'? — There is, however, no difficulty, as the latter ruling was [in a case where the slaughterer was working] gratuitously whereas the former ruling applies [where the slaughterer works] for hire,<span class="x" onmousemove="('comment',' V. p. 580, n. 10. ');">³¹</span> exactly as R. Zera said: If one wants the slaughterer to become liable to him,<span class="x" onmousemove="('comment',' Were the slaughter not carried out effectively. ');">³²</span>

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