Reference for Mikvaot 2:3
סְפֵק מַיִם שְׁאוּבִין שֶׁטִּהֲרוּ חֲכָמִים, סָפֵק נָפְלוּ, סָפֵק לֹא נָפְלוּ, אֲפִלּוּ נָפְלוּ, סָפֵק יֶשׁ בָּהֶם אַרְבָּעִים סְאָה סָפֵק אֵין בָּהֶם, שְׁנֵי מִקְוָאוֹת, אֶחָד יֶשׁ בּוֹ אַרְבָּעִים סְאָה וְאֶחָד אֵין בּוֹ, נָפַל לְאַחַד מֵהֶן וְאֵינוֹ יוֹדֵעַ לְאֵיזֶה מֵהֶן נָפַל, סְפֵקוֹ טָהוֹר, מִפְּנֵי שֶׁיֶּשׁ לוֹ בַמֶּה יִתְלֶה. הָיוּ שְׁנֵיהֶם פְּחוּתִים מֵאַרְבָּעִים סְאָה, וְנָפַל לְאַחַד מֵהֶם וְאֵינוֹ יוֹדֵעַ לְאֵיזֶה מֵהֶן נָפַל, סְפֵקוֹ טָמֵא, מִפְּנֵי שֶׁאֵין לוֹ בַמֶּה יִתְלֶה:
Cases of uncertainty regarding drawn waters are considered pure by the Sages. [For example,] if it is uncertain whether they [the drawn waters] fell or did not fall [into a <i>mikveh</i>]; even if they did fall, if there is an uncertainty as to whether they [the waters of the <i>mikveh</i>] contained forty se'ah or did not; or if there were two <i>mikva'ot</i>, one mikveh containing forty <i>se'ah</i> and one not, and [drawn waters] fell into one of them and one does not know which it fell into, [in all these cases] its uncertainty is pure, because one has something upon which to hang [a presumption of purity]. If both [<i>mikvaot</i>] contained fewer than forty <i>se'ah</i>, and [drawn waters] fell into one of them and one does not know which it fell into, its uncertainty is impure [i.e. such a case is uncertain and resolved as impure], since one has nothing upon which to hang [a presumption of purity].