Tractate Mikvaot - Chapter 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.