And a pure man shall gather the ashes of the heifer and place them outside the camp in a pure place; and it shall be for the congregation of the children of Yisrael for a keeping, for water of sprinkling; it is a chatas.
Torah Temimah: Sifrei — Why is “for water of sprinkling” said? One might have thought that melachah disqualifies only the para itself; from where do we know [it also disqualifies] the water? Therefore it says “and it shall be for a keeping.” Or [one might think] even after they have been mixed; therefore it says “for water of sprinkling” — once they are already for water of sprinkling [the disqualification no longer applies in the same way].
English: Why is it said? One might have thought that melachah disqualifies only the para itself; from where [do we know it also applies] to the water? Therefore it says “and it shall be for a keeping.” Or even after they have been mixed; therefore it says “for water of sprinkling” — they are already for water of sprinkling.
Extension of the melachah disqualification from the para to the water, with a limit after mixing.
Extraneous labor disqualifies not only the para itself but also the water set aside for the mei niddah, until the ashes and water have been mixed.