Reference for Chullin 257:22

טומאתה לאו דאורייתא דאי סלקא דעתך דאורייתא מצינו לאוכלין שמטמאין טומאה חמורה

The uncleanness [in such a case I say] is not decreed by the law of the Torah; for should you say it is so by the law of the Torah then we should have a case of foodstuffs being able to convey the graver uncleanness [later on]! - [No.

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