עֶרְוַ֥ת אֵֽשֶׁת־אָבִ֖יךָ לֹ֣א תְגַלֵּ֑ה עֶרְוַ֥ת אָבִ֖יךָ הִֽוא׃ (ס)
The nakedness of thy father’s wife shalt thou not uncover: it is thy father’s nakedness.
Rashi on Leviticus
ערות 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).