Mishneh Torah — Positive Mitzvot מצוות עשה, Chapter 99
The full Hebrew text of Mishneh Torah, Positive Mitzvot, Chapter 99, with English translation by Maimonides (Rambam).
להיות הנידה טמאה ומטמאה לאחרים.
For [a woman in the] niddah state to be impure and to impart impurity to others.