Sefer HaChinuch 181 ספר החינוך קפ״א

The commandment of the matter of the impurity of the menstruant, that is impure and renders impure: That the menstruant be impure and render others impure, as it is stated (Leviticus 15:19), “and anyone who touches her shall be impure until the evening.”

We will write the root of the commandment and a little of its details — as per our custom — in the negative commandment of the menstruant in the Order of Acharei Mot (Sefer HaChinukh 207), with God’s help.

And [it] is practiced in every place and at all times. And one who transgresses it and does not act with her [according to her] impurity — and likewise if she, herself, acts [according to] the practice of purity (and not impurity) — has violated this positive commandment, besides the punishment that there is with it, as we shall write below with God’s help.