Mishnà su Numeri 15:5

Hillel says: “A hin full of drawn water renders the mikweh unfit.” (However, man must speak in the language of his teacher.) And Shammai says: “Nine kavs.” But the Sages say: “Neither according to the opinion of this one nor according to the opinion of this one;” But when two weavers from the dung-gate which is in Jerusalem came and testified in the name of Shemaiah and Avtalion, “Three logs of drawn water render the mikweh unfit,” the Sages confirmed their statement.