Temurah 12B

והן טהורין, שהשאיבה מטהרת ברבייה ובהמשכה.

and in this manner all 40 se’a are ritually pure. The reason is that drawn water is purified by a majority of fit water that was already present in the mikveh and by the drawn water flowing into the mikveh. This is the meaning of the Mishnah’s statement that drawn water invalidates the mikveh only according to calculation: One calculates the amount of drawn water that flowed into the mikveh, as the mikveh is invalid only if most of the 40 se’a is of that drawn water.

מכלל דרבנן סברי דברבייה ובהמשכה לא? אלא דכי אתא רבין אמר רבי יוחנן: שאובה שהמשיכוה כולה ⁠— טהורה. מני? לא רבנן ולא רבי אליעזר!

The Talmud asks: If this is the opinion of R' Eliezer ben Ya’akov alone, one must conclude by inference that the Rabbis hold that the mikveh is not fit even by a majority of fit water and by the flowing of the drawn water. But if so, consider that which Ravin said when he came from Eretz Yisrael to Babylonia, namely, that R' Yoḥanan said: A mikveh that consists in its entirety of drawn water that one made flow into it is pure. In accordance with whose opinion is this? It is not in accordance with the opinion of the Rabbis, who maintain that drawn water invalidates the mikveh even if it flowed into it, and it is not in accordance with the opinion of R' Eliezer ben Ya’akov, who holds that drawn water invalidates the mikveh if it constitutes a majority of the 40 se’a.

אלא אמר רבה: לפי חשבון כלים, ויוסף בן חוני היא.

Rather, Rabba says: The Mishnah’s ruling, that drawn water invalidates the mikveh only according to calculation, is not referring to drawn water that flowed. Instead, it means according to the calculation of the number of vessels from which the drawn water was directly poured into the mikveh, and this is in accordance with the opinion of Yosef ben Ḥoni.