Parashas Metzora · Passuk 15:31
Chabad.org — English translation + Rashi
Torah Temima points to R’ Yoshiyah.
English: “And you shall separate …” — R’ Yoshiyah said: “And you shall separate the children of Israel from their impurity” — from here is a warning to the children of Israel that they separate from their wives near the time of their expected period.
Separation near expected period
Chazal derive a practical warning: separate near the expected onset of niddah so as not to stumble into the prohibition during relations.