Parashas Metzora · Passuk 15:23
Chabad.org — English translation + Rashi
Torah Temima points to Tanna de-vei R’ Yishmael (via Toras Kohanim / Bavli context).
English: “On which she sits” — the school of R’ Yishmael taught: A person should always speak in clean language, for regarding the zav Scripture called it merchav and regarding a woman it called it moshav.
Lashon nekiyah
Chazal derive from the Torah’s own diction a standing rule of refined speech.