Reference for Pesachim 31:11

ת"ש (ויקרא יא, לד) וכל משקה אשר ישתה בכל כלי יטמא מאי יטמא הכשיר

He holds that the uncleanness of liquids themselves is Scriptural, [but] in respect of defiling others, Rabbinical; and when did the Rabbis decree? In respect of liquids in general, but in respect of the liquids of the slaughter-house there was no decree; again, when did the Rabbis refrain from decreeing [concerning the liquids of the slaughter-house]?

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