Reference for Kelim 25:3
מִדּוֹת יַיִן וָשֶׁמֶן, וְזוֹמָא לִסְטְרָא, וּמְסַנֶּנֶת שֶׁל חַרְדָּל, וּמְשַׁמֶּרֶת שֶׁל יַיִן, יֶשׁ לָהֶן אֲחוֹרַיִם וָתוֹךְ, דִּבְרֵי רַבִּי מֵאִיר. רַבִּי יְהוּדָה אוֹמֵר, אֵין לָהֶם. רַבִּי שִׁמְעוֹן אוֹמֵר, יֶשׁ לָהֶם. נִטְמְאוּ מֵאֲחוֹרֵיהֶן, מַה שֶּׁבְּתוֹכָן טָהוֹר, וְצָרִיךְ לְהַטְבִּיל:
Measuring cups for wine or oil, and a fork-ladle, and a mustard-strainer, and a wine-filter all have [a distinction, with regard to purity status, between their] insides and outsides, according to Rabbi Meir. Rabbi Yehuda says: they do not have [such a distinction]. Rabbi Shimon says: they do have [such a distinction], for if they are rendered impure from their outsides, that which is inside them is pure; and one is required to immerse [the vessel in its entirety, in order to purity it].