Parashas Metzora · Passuk 15:26
Chabad.org — English translation + Rashi
Torah Temima points to Toras Kohanim.
English: “Like the bed of her niddah it shall be for her” — it was taught: From where [do we know] that she counts one against one (shomeres yom keneged yom)? Scripture teaches “it shall be for her.”
Shomeres yom keneged yom
Chazal derive the rule that a minor zavah (one or two days) observes the following day as a clean day corresponding to the sighting day.