Reference for Mikvaot 6:7
עֵרוּב מִקְוָאוֹת, כִּשְׁפוֹפֶרֶת הַנּוֹד, כְּעָבְיָהּ וְכַחֲלָלָהּ, כִּשְׁתֵּי אֶצְבָּעוֹת חוֹזְרוֹת לִמְקוֹמָן. סָפֵק כִּשְׁפוֹפֶרֶת הַנּוֹד, סָפֵק שֶׁאֵינָהּ כִּשְׁפוֹפֶרֶת הַנּוֹד, פְּסוּלָה, מִפְּנֵי שֶׁהִיא מִן הַתּוֹרָה. וְכֵן כַּזַּיִת מִן הַמֵּת וְכַזַּיִת מִן הַנְּבֵלָה וְכָעֲדָשָׁה מִן הַשֶּׁרֶץ. כֹּל שֶׁיַּעֲמֹד כִּשְׁפוֹפֶרֶת הַנּוֹד, מְמַעֲטָהּ. רַבָּן שִׁמְעוֹן בֶּן גַּמְלִיאֵל אוֹמֵר, כֹּל שֶׁהוּא מִבְּרִיַּת הַמַּיִם, טָהוֹר:
<i>Mikvaot</i> are [considered] mixed [when they are connected by an opening] the size of the tube of a flask, alike to it in its width, and alike to it in [the length of] its hollow, such that two fingers can be rotated in place [i.e. its diameter should be the length of two fingers]. If there is an uncertainty as to whether it [the opening] was the size of the tube of a flask or was not the size of the tube of a flask, it is invalid because it is [a requirement] from the Torah [to immerse in a valid <i>mikveh</i>, and therefore we rule stringently on the matter]. Similarly [we rule stringently in a situation of uncertainty] regarding an olive's bulk of a corpse, and an olive's bulk of a carcass, and a lentil's bulk of a vermin. Anything that will stand in [an opening] the size of the tube of a flask, reduces it [from being that size, and thus from being able to be a connection between <i>mikvaot</i>]. Rabbi Shimon ben Gamliel says: any thing that is created out of the water is pure [i.e. does not reduce the size of the opening].