Midrash su Levitico 20:18

1) (Vayikra 20:18) ("And a man who lies with a woman in her flow and reveals her nakedness — he has bared her fountain and she has revealed the fountain of her blood; and both of them shall be cut off from the midst of their people.") "And a man": to exclude a minor. "who lies with a woman in her flow" (davah): "davah" is "niddah," as it is written (Vayikra 15:33) "And she who flows (davah) with her niddah flow, and he who flows with his discharge."

2) "if she will have": from the pronouncement on. "a flow": I might think even if she flows from any place she is tamei; it is, therefore, written (Vayikra 20:18) "and she has revealed the source of her blood." This teaches us about (her) blood that (it causes uncleanliness) only if it comes from the source.

(Lev. 15:25) “And when a woman has had a discharge of blood for many days.” Let our master instruct us: Is it permitted for a menstruant to sleep in the same bed with her husband, when he is in his clothes and she is in her clothes, one on one side and one on the other?³²Shab. 13a. Thus have our masters taught: It is forbidden [for them] to lie down [together], because one does not one put a breech before a proper man, and certainly not before the thief. Thus the sages compare the matter to a fire in the straw; and it says (in Lev. 18:19), “And you shall not come near a women during her period of menstrual uncleanness.” [This is] to teach you that the Holy One, blessed be He, warns Israel about sanctification and about purity, lest they become unclean through their wives when they are menstruating; for whoever has intercourse with his wife when she is menstruating is under sentence of being cut off, as stated (in Lev. 20:18), “And if a man lies with a woman [when she is] unwell…, they both shall be cut off.”