וְצִוָּה֙ הַכֹּהֵ֔ן וְשָׁחַ֖ט אֶת־הַצִּפּ֣וֹר הָאֶחָ֑ת אֶל־כְּלִי־חֶ֖רֶשׂ עַל־מַ֥יִם חַיִּֽים׃
Ferner gebiete der Priester, dass man den einen Vogel schlachte in ein irdenes Gefäß, über lebendigem [frischem] Wasser.
Rashi on Leviticus
אל מים חיים OVER RUNNING WATER — He first places it (the water) in the vessel but only, just enough that the bird’s blood may remain discernible in it. And how much is this? The fourth of a log (Sotah 16b).
Tur HaArokh
אל כלי חרס, “into an earthen vessel.” Allegorically speaking, the person preparing to rejoin society is to humble himself, like broken earthenware.
Siftei Chakhamim
First in a vessel. Because at first it is written, “in an earthenware vessel,” and afterwards it is written, “over running water.” How could that be? Rashi answers: Both are true: He [first] places [the water in a vessel]...