Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 334
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 334, with English translation by Maimonides (Rambam).
שלא לגלות ערוות בת הבן, שנאמר "ערוות בת בנך" (ויקרא יח,י).
Not to have intimate relations with one's son's daughter, as [Leviticus 18:10] states: "[Do not commit] incest with your son's daughter."