Positive commandment · Mitzvat Aseh #105 of 613
For semen to impart impurity
להיות שכבת זרע מטמאה
The verse · Vayikra 15:16
וְאִישׁ כִּי־תֵצֵא מִמֶּנּוּ שִׁכְבַת־זָרַע וְרָחַץ בַּמַּיִם אֶת־כׇּל־בְּשָׂרוֹ וְטָמֵא עַד־הָעָרֶב
“And if the semen goes out from a man, then he shall bathe all his flesh in water, and be unclean until evening.”
The Rambam’s words
For semen to impart impurity.
להיות שכבת זרע מטמאה.
Why · Sefer HaChinuch #180
Matter of the impurity of semen, which is impure and renders impure
“It is from the roots of the commandment [that] since this matter only happens from the angle of thoughts of physical desires, the perfect Torah informed us that the body is called impure with them.”
Who & when
“And the impurity of semen is practiced in every place and at all times.”
Where in the Mishneh Torah
Hilchot Other Sources of Defilement הלכות שאר אבות הטומאות