Rashi on Deuteronomy 24:4/רש"י על דברים כ״ד:ד׳

1אחרי אשר הטמאה [HER FIRST HUSBAND … MAY NOT TAKE HER AGAIN TO BE HIS WIFE] AFTER THAT SHE IS DEFILED — these words are intended to include in the prohibition a faithless wife (סוטה) who had been in seclusion with another man (Sifrei Devarim 270:10; Yevamot 11b).