Sefer HaMitzvot, Positive Commandments
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היא שצונו להיות הזב טמא, ומצוה זו כוללת משפט הדברים שיהיה בהן זב ותאר היותו מטמא לזיבתו. (מצורע, טהרה הלכות טומאת מטמאי מושב פ"ו):
Sefaria Edition 2021, Translated by Rabbi Francis Nataf
That is that He commanded us that a zav be impure. And this commandment includes the laws of the things that would make him a zav and the description of his transmitting his [impurity]. (See Parashat Metzora; Mishneh Torah, Those Who Defile Bed or Seat 6.)