Reference for Pesachim 28:7

ממאי דילמא לטמא אחרים מדרבנן אי מדרבנן מאי איריא באב הטומאה אפילו בראשון ושני נמי תחלה הוי

Raba said: This proves that R'Akiba holds, The uncleanness of liquids in respect of defiling others is Scriptural; for if you should think that it is Rabbinical [only], then consider: how does this lamp affect the oil? If by rendering that itself unfit, surely it is already unfit?<span class="x" onmousemove="('comment',' And what does it matter whether it is of the third degree or of the first? Hence we must assume that it can now contaminate even by Scriptural law, which it could not do before.');">⁷</span>

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