Commentary for Avodah Zarah 150:8

Bar Kapara derives the halakhah that vessels bought from a Gentile must be immersed in the mikveh from the use of the word “the waters of lustration” which literally translates as “the waters of the menstruant (niddah). Just as a menstruant must immerse in a mikveh, so too must these vessels be immersed in the mikveh.