Reference for Chullin 243:31

מאן דמתני לה ארישא כ"ש אסיפא ומאן דמתני אסיפא אבל ארישא טומאה דאורייתא:

Others refer this [dictum of Ulla's] to the later clause of our Mishnah, viz. , IF ANY OF THESE [SKINS] WAS TANNED OR TRAMPLED UPON AS MUCH AS [WAS USUAL] FOR TANNING, IT BECOMES CLEAN, EXCEPTING THE SKIN OF A MAN.

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