Reference for Tahorot 8:7
אֲחוֹרֵי כֵלִים שֶׁנִּטְמְאוּ בְמַשְׁקִים, רַבִּי אֱלִיעֶזֶר אוֹמֵר, מְטַמְּאִין אֶת הַמַּשְׁקִין וְאֵין פּוֹסְלִין אֶת הָאֳכָלִין. רַבִּי יְהוֹשֻׁעַ אוֹמֵר, מְטַמְּאִין אֶת הַמַּשְׁקִין וּפוֹסְלִין אֶת הָאֳכָלִין. שִׁמְעוֹן אֲחִי עֲזַרְיָה אוֹמֵר, לֹא כָךְ וְלֹא כָךְ, אֶלָּא מַשְׁקִין שֶׁנִּטְמְאוּ בַאֲחוֹרֵי הַכֵּלִים, מְטַמְּאִין אֶחָד וּפוֹסְלִין אֶחָד. הֲרֵי זֶה אוֹמֵר, מְטַמְּאֶיךָ לֹא טִמְּאוּנִי, וְאַתָּה טִמֵּאתָנִי:
The outsides of vessels which were rendered impure by liquids, Rabbi Eliezer says: they can render liquids impure but do not invalidate [<i>terumah</i>] foods [by rendering htem impure]. Rabbi Yehoshua says: they can make liquids impure and invalidate [<i>terumah</i>] foods. Shimon the brother of Azaryah says: neither this or that, but rather liquids that became impure from the outsides of vessels render impure one [item upon contact, rendering it a second degree level of impurity], and [that item, in turn, is able to] invalidate one [more item, rendering it a third degree level of impurity, which invalidates <i>terumah</i> foods]. Behold it [is as though the <i>terumah</i> food] says [to the liquids], "that which renders you impure [i.e. the outsides of vessels] does not impurify me, yet you have rendered me impure!"