Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 338
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 338, with English translation by Maimonides (Rambam).
שלא לגלות ערוות אישה ובת בנה, שנאמר "את בת בנה" (ויקרא יח,יז).
Not to have intimate relations with a woman and her son's daughter, as [Leviticus, ibid.] continues: "...her son's daughter...."