Commentary for Avodah Zarah 74:2

ועל משקה בי מטבחיא דכן: מאי דכן רב אמר דכן ממש ושמואל אמר דכן מלטמא אחרים אבל טומאת עצמן יש בהן

“That the flow [of blood and water] from the place of slaughter [in the Temple] is pure.” What means “pure” mean? Rav said: It is actually pure. But Shmuel said: It is pure in the sense that it does not render other things impure but in itself there was impurity.

Daf Shevui to Avodah Zarah