Reference for Eduyot 2:8
שְׁלֹשָׁה דְבָרִים אָמַר רַבִּי עֲקִיבָא, עַל שְׁנַיִם הוֹדוּ לוֹ וְעַל אֶחָד לֹא הוֹדוּ לוֹ. עַל סַנְדָּל שֶׁל סַיָּדִים, שֶׁהוּא טָמֵא מִדְרָס. וְעַל שְׁיָרֵי תַנּוּר אַרְבָּעָה, שֶׁהָיוּ אוֹמְרִים שְׁלֹשָׁה. וְהוֹדוּ לוֹ. וְעַל אֶחָד לֹא הוֹדוּ לוֹ, עַל כִּסֵּא שֶׁנִּטְּלוּ שְׁנַיִם מֵחִפּוּיָיו זֶה בְּצַד זֶה, שֶׁרַבִּי עֲקִיבָא מְטַמֵּא וַחֲכָמִים מְטַהֲרִין:
R. Akiva said three things. About two, they (the sages) concurred with him; about one, they did not concur with him. (He said) about the shoe of those who work in lime [(The lime workers wore wooden shoes to protect their feet from being burnt by the lime)], that it (the shoe) contracts midras ("treading"-) uncleanliness [if worn by a zav]. And (he said) about what was left of an oven [that was very large and became tamei and was afterwards broken, that it does not become tahor unless what was left over was less than] four [(tefachim) in height. (An "oven" in the Mishnah was made like a big pot without a bottom, connected with clay to the ground, which was its "bottom.")] For they (the sages) (originally) had said ("less than) three," and (now) they concurred with him.