Positive commandment · Mitzvat Aseh #104 of 613
For a zav to impart impurity
להיות הזב מטמא
The verse · Vayikra 15:3-4
וְזֹאת תִּהְיֶה טֻמְאָתוֹ בְּזוֹבוֹ רָר בְּשָׂרוֹ אֶת־זוֹבוֹ אוֹ־הֶחְתִּים בְּשָׂרוֹ מִזּוֹבוֹ טֻמְאָתוֹ הִוא כׇּל־הַמִּשְׁכָּב אֲשֶׁר יִשְׁכַּב עָלָיו הַזָּב יִטְמָא וְכׇל־הַכְּלִי אֲשֶׁר־יֵשֵׁב עָלָיו יִטְמָא
“And this shall be his uncleanness in his issue: whether his flesh run with his issue, or his flesh be stopped from his issue, it is his uncleanness. Every bed, whereon he that has the issue lies down, is unclean: and everything, whereon he sits, shall be unclean.”
The Rambam’s words
For a zav to impart impurity.
להיות הזב מטמא.
Why · Sefer HaChinuch #178
Matter of impurity of a zav (one with a discharge) to be impure and to render impure
“It is from the roots of the commandment [that it is] since G-d, blessed be He, distanced us greatly from the excesses and commanded us to be holy and righteous regarding food and drink and in all other matters of man.”
Who & when
“And this matter that one with a discharge is called impure is practiced in every place and at all times.”
Where in the Mishneh Torah
Hilchot Those Who Defile Bed or Seat הלכות מטמאי משכב ומושב