Rashi on Leviticus 18:8/רש"י על ויקרא י״ח:ח׳

1ערות אשת אביך THE NAKEDNESS OF THY FATHER'S WIFE [SHALT THOU NOT UNCOVER] — This is intended to include in the prohibition of union with one's father's wife already mentioned v. 7. even the doing of this after his death (Sanhedrin 54a).