Parashas Metzora · Passuk 15:23
Chabad.org — English translation + Rashi
Torah Temima points to Toras Kohanim.
English: “On which she sits” — she, and not her blood.
Person vs. blood as source of midras
Chazal teach that only the niddah herself makes her bed an av hatumah for people and clothes; her blood alone makes a bed only a rishon, conveying impurity by contact and carrying but not as full midras of her body.