Mishnayos Mikvaos Perek 2 Mishnah 1

An unclean man who went down to immerse himself: If it is doubtful whether he did immerse himself or not; And even if he did immerse himself, it is doubtful whether the mikveh contained forty seahs or not; And if there were two mikvehs, one containing forty seahs but the other not containing forty seahs, and he immersed himself in one of them but he does not know in which of them he immersed himself, In such a doubt he is unclean.

הַטָּמֵא שֶׁיָּרַד לִטְבֹּל, סָפֵק טָבַל סָפֵק לֹא טָבַל, אֲפִלּוּ טָבַל, סָפֵק יֶשׁ בּוֹ אַרְבָּעִים סְאָה סָפֵק אֵין בּוֹ. שְׁנֵי מִקְוָאוֹת, אֶחָד יֶשׁ בּוֹ אַרְבָּעִים סְאָה וְאֶחָד שֶׁאֵין בּוֹ, טָבַל בְּאַחַד מֵהֶם וְאֵינוֹ יוֹדֵעַ בְּאֵיזֶה מֵהֶן טָבַל, סְפֵקוֹ טָמֵא: