Positive commandment · Mitzvat Aseh #106 of 613
For a zavah to impart impurity
להיות זבה מטמאה
The verse · Vayikra 15:19
וְאִשָּׁה כִּי־תִהְיֶה זָבָה דָּם יִהְיֶה זֹבָהּ בִּבְשָׂרָהּ שִׁבְעַת יָמִים תִּהְיֶה בְנִדָּתָהּ וְכׇל־הַנֹּגֵעַ בָּהּ יִטְמָא עַד־הָעָרֶב
“And if a woman have an issue, and her issue in her flesh be blood, she shall be seven days in her menstrual separation: and whoever touches her shall be unclean until evening.”
The Rambam’s words
For a zavah to impart impurity.
להיות זבה מטמאה.
Who & when
“And the rest of its details are in Tractate Zavim, and a few of the laws of discharge are also in Tractate Niddah. (See Tur, Yoreh Deah 183.) And in the negative commandment of the menstruant in the Order of Acharei Mot (Sefer HaChinukh 207), we will write, with G-d’s help, that which is practiced today.”
Where in the Mishneh Torah
Hilchot Those Who Defile Bed or Seat הלכות מטמאי משכב ומושב