Mishnah Kelim 25:6-7

כַּנֵּי כֵלִים, וְאֹגְנֵיהֶם, וְאָזְנֵיהֶם, וִידוֹת הַכֵּלִים הַמְקַבְּלִים, שֶׁנָּפְלוּ עֲלֵיהֶן מַשְׁקִין, מְנַגְּבָן וְהֵם טְהוֹרִים. וּשְׁאָר כָּל הַכֵּלִים (שֶׁאֵינָם יְכוֹלִין לְקַבֵּל רִמּוֹנִים), שֶׁאֵין לָהֶם אֲחוֹרַיִם וָתוֹךְ, שֶׁנָּפְלוּ מַשְׁקִין עַל מִקְצָתוֹ, כֻּלּוֹ טָמֵא. כְּלִי שֶׁנִּטְמְאוּ אֲחוֹרָיו בְּמַשְׁקִין, אֲחוֹרָיו טְמֵאִים, תּוֹכוֹ וְאֹגְנוֹ וְאָזְנוֹ וְיָדָיו טְהוֹרִין. נִטְמָא תוֹכוֹ, כֻּלּוֹ טָמֵא:

The bases, rims, hangers or handles of vessels that have a receptacle upon which an unclean liquid fell, one dries them and they remain clean. But [if unclean liquid fell] on any part of any other vessel which cannot hold pomegranates (or in which no distinction is made between its outer and inner sides), the whole becomes unclean. If the outer side of a vessel contracted uncleanness from a liquid, only its outer side is unclean but its inner side, rim, hanger and handles remain clean. If its inner side contracted uncleanness the whole is unclean.

כָּל הַכֵּלִים יֶשׁ לָהֶן אֲחוֹרַיִם וָתוֹךְ, וְיֶשׁ לָהֶם בֵּית צְבִיעָה. רַבִּי טַרְפוֹן אוֹמֵר, לַעֲרֵבָה גְדוֹלָה שֶׁל עֵץ. רַבִּי עֲקִיבָא אוֹמֵר, לְכוֹסוֹת. רַבִּי מֵאִיר אוֹמֵר, לְיָדַיִם הַטְּמֵאוֹת וְהַטְּהוֹרוֹת. אָמַר רַבִּי יוֹסֵי, לֹא אָמְרוּ אֶלָּא לְיָדַיִם הַטְּהוֹרוֹת בִּלְבָד:

All vessels have outer and inner sides and have a part by which they are held. Rabbi Tarfon says: this applies only to a large wooden [kneading] trough. Rabbi Akiva says: it applies to cups. Rabbi Meir says: it applies to unclean and clean hands. Rabbi Yose says: they said this only concerning clean hands.