Negative commandment · Lo Ta’aseh #346 of 613
Not to have intimate relations with a woman in the niddah state
שלא לגלות ערוות נידה
The verse · Vayikra 18:19
וְאֶל־אִשָּׁה בְּנִדַּת טֻמְאָתָהּ לֹא תִקְרַב לְגַלּוֹת עֶרְוָתָהּ
“Also thou shalt not approach to a woman in the impurity of her menstrual flow, to uncover her nakedness.”
The Rambam’s words
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."
שלא לגלות ערוות נידה, שנאמר "ואל אישה, בנידת טומאתה--לא תקרב" (ויקרא יח,יט).
Why · Sefer HaChinuch #207
To not have intercourse with a menstruant woman
“From the roots of the commandment, [there is a need] for a preface: There is no doubt that all of the sicknesses of people come either because of an excess in the body or from the angle of a deficit that happens from whatever reason it might be.”
Who & when
“And this prohibition is practiced in every place and at all times.”
Where in the Mishneh Torah
Hilchot Forbidden Intercourse הלכות איסורי ביאה