Parashas Acharei Mos · Passuk 18:27
Chabad.org — English translation + Rashi
Torah Temima points to Yevamos 21a.
English: Hint for secondary (rabbinic) arayot from the Torah: “these” (ha-el) implies hard ones, from which it follows there are softer ones — the sheniyot.
Scriptural hint for sheniyot
The wording “these to’evot” supports the existence of a secondary tier of forbidden relations enacted by the Sages.