Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 339
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 339, with English translation by Maimonides (Rambam).
שלא לגלות ערוות אישה ובת בתה, שנאמר "ואת בת בתה, לא תיקח" (ויקרא יח,יז).
Not to have intimate relations with a woman and her daughter's daughter, as [Leviticus, ibid.] continues: "...her daughter's daughter,... you shall not take."