Rashi on Numbers 5:17/רש"י על במדבר ה׳:י״ז

1מים קדשים HOLY WATER — i.e., water that has become holy through being in the laver. — The water was taken from the laver because that was made of the copper mirrors of the women who had gathered (Exodus 38:8) [at the entrance to the Tent of Meeting] and this woman deviated from their chaste ways. Because they had cherished their husbands’ love in Egypt (cf. Rashi on Exodus 38:8), while this woman depravedly gave herself over to another, she was to be examined through it (Bamidbar Rabbah 9 :14).

2בכלי חרש IN AN EARTHEN VESSEL — She gave the adulterer excellent wine to drink in valuable goblets, therefore she shall drink the bitter water in a common earthenware cup. (Sotah 9a.)