Mishneh Torah — Negative Mitzvot מצוות לא תעשה, Chapter 346
The full Hebrew text of Mishneh Torah, Negative Mitzvot, Chapter 346, with English translation by Maimonides (Rambam).
שלא לגלות ערוות נידה, שנאמר "ואל אישה, בנידת טומאתה--לא תקרב" (ויקרא יח,יט).
Not to have intimate relations with a woman in the niddah state, as [Leviticus 18:19] states: "Do not come close to a woman in the niddah state of impurity."