Rashi on Numbers 25:18/רש"י על במדבר כ״ה:י״ח

1כי צררים הם לכם וגו׳ על דבר פעור FOR THEY SHOWED THEMSELVES YOUR ENEMIES etc. IN THE MATTER OF PEOR — i.e., in that they abandoned their daughters to prostitution in order to lead you astray after Peor; but He did not command them to destroy the Moabites — on account of Ruth who would later on issue from them, just as we state in Bava Kamma 38b.