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: Yoma 42b — From here [we learn] that the gathering of the ashes and the filling of the water for the purification require shemirah (guarding).
English: From here [we learn] that the gathering of the ashes and the filling of the water for the purification require shemirah (guarding).
Derivation of the shemirah requirement from “lemishmeres.”
Both the collection of the para ashes and the drawing of the water that will later be mixed with them must be performed under intentional guarding against impurity.