Parashas Acharei Mos · Passuk 18:19
Chabad.org — English translation + Rashi
Torah Temima points to Shevuos 18b.
English: “Do not approach to uncover” — this is the Torah’s explicit warning to one who cohabits with a niddah.
Biblical azharah for bo’el niddah
The verse supplies the formal prohibition (azharah) required for the penalty structure of relations with a niddah.