Sefer HaMitzvot, Positive Commandments
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היא שצונו להיות זבה טמאה, ומצוה זו כוללת דקדוק הדברים שבהם תהיה הזבה מטמאה זולתה אחר שהגיע לה הזיבות. (מצורע, טהרה הלכות טומאת מטמאי משכב פ"ה):
Sefaria Edition 2021, Translated by Rabbi Francis Nataf
That is that He commanded us that a zavah be impure. And this commandment includes the exact ways through which the zavah transmits impurity to others after [it] has come to her. (See Parashat Metzora; Mishneh Torah, Those Who Defile Bed or Seat 5.)