Sefer HaMitzvot, Positive Commandments
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היא שצונו להיות שכבת זרע טמא, ומצוה זו כוללת משפטי שכבת זרע. (מצורע, טהרה הלכות טומאה שאר טומאות פ"ה):
Sefaria Edition 2021, Translated by Rabbi Francis Nataf
That is that He commanded us that semen be impure. And this commandment includes the regulations of [the impurity of] semen. (See Parashat Metzora; Mishneh Torah, Other Sources of Defilement 5.)