Reference for Niddah 109:42

ומה טעם אמרו טמא גזרה שמא יעשה אדם עור אביו ואמו שטיחין

— Surely in connection with this ruling it was stated: 'Ulla said, 'Pentateuchally the skin of a human being is clean, and what is the reason why they ruled it to be unclean? It is a preventive measure against the possibility that a person might use the skins of his father and mother as spreads for an ass.'

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