Reference for Bekhorot 75:28

הלכך שויוה רבנן כטומאה דכלי חרס ולא שויוה רבנן כטומאה דאורייתא דנפשיה

Since where there is an inside, the Rabbis differentiated, it will indeed be known that where there is no inside the uncleanness is a rabbinic enactment [and that therefore terumah must not be burnt in consequence of it].

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