Reference for Sotah 5:2
בּוֹ בַּיּוֹם דָּרַשׁ רַבִּי עֲקִיבָא, (ויקרא יא) וְכָל כְּלִי חֶרֶשׂ אֲשֶׁר יִפֹּל מֵהֶם אֶל תּוֹכוֹ כֹּל אֲשֶׁר בְּתוֹכוֹ יִטְמָא, אֵינוֹ אוֹמֵר טָמֵא אֶלָּא יִטְמָא, לְטַמֵּא אֲחֵרִים, לִמֵּד עַל כִּכָּר שֵׁנִי שֶׁמְּטַמֵּא אֶת הַשְּׁלִישִׁי. אָמַר רַבִּי יְהוֹשֻׁעַ, מִי יְגַלֶּה עָפָר מֵעֵינֶיךָ, רַבָּן יוֹחָנָן בֶּן זַכַּאי, שֶׁהָיִיתָ אוֹמֵר, עָתִיד דּוֹר אַחֵר לְטַהֵר כִּכָּר שְׁלִישִׁי, שֶׁאֵין לוֹ מִקְרָא מִן הַתּוֹרָה שֶׁהוּא טָמֵא. וַהֲלֹא עֲקִיבָא תַּלְמִידְךָ מֵבִיא לוֹ מִקְרָא מִן הַתּוֹרָה שֶׁהוּא טָמֵא, שֶׁנֶּאֱמַר, כֹּל אֲשֶׁר בְּתוֹכוֹ יִטְמָא:
On that day [the day Rabban Gamliel was deposed from the position of Prince] Rabbi Akiva expounded, "any earthenware vessel into which any of them fall, whatever is within it shall become impure" (Leviticus 11:33) It does not say "impure" but "becomes impure" -- thus it makes others impure. This teaches that a loaf of second-degree impurity can make impure a loaf to the third-degree. Rabbi Yehoshua said, who will remove the dirt from your eyes Rabban Yochanan ben Zakkai? You used to say that in the future another generation will purify a third-degree loaf, because there is no verse from the Torah that makes it impure. Now behold, Akiva your student brings a verse from the Torah that it is impure! As it says, "all that is in it becomes impure."