Reference for Mikvaot 2:4
רַבִּי אֱלִיעֶזֶר אוֹמֵר, רְבִיעִית מַיִם שְׁאוּבִין בַּתְּחִלָּה, פּוֹסְלִין אֶת הַמִּקְוֶה, וּשְׁלשָׁה לֻגִּין עַל פְּנֵי הַמַּיִם. וַחֲכָמִים אוֹמְרִים, בֵּין בַּתְּחִלָּה בֵּין בַּסּוֹף, שִׁעוּרוֹ שְׁלשָׁה לֻגִּין:
Rabbi Eliezer says: A <i>revi'it</i> [a specific measure of volume, a quarter of a <i>log</i>] of drawn waters render a <i>mikveh</i> invalid at the outset [i.e. if the waters were in the pit before the valid <i>mikveh</i> waters were added, as do as three <i>log</i> [of drawn waters which were added] on the surface of the [valid] waters. And the Sages say: whether at the outset [before there were any waters in the pit] or at the end [after valid waters amounting to less than forty <i>se'ah</i> were added], the measure [for the minimum amount of drawn waters to invalidate a <i>mikveh</i>] is three <i>log</i>.