Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 335
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 335, with English translation by Maimonides (Rambam).
שלא לגלות ערוות בת הבת, שנאמר "או בת בתך, לא תגלה ערוותן" (ויקרא יח,י).
Not to have intimate relations with one's daughter's daughter, as [Leviticus, ibid.] continues: "...and do not commit incest with your daughter's daughter."