Parashas Metzora · Passuk 15:28
Chabad.org — English translation + Rashi
Torah Temima points to Shmuel (via Rav Chinena bar Kahana).
English: “And she shall count for herself” — it was taught: Rav Chinena bar Kahana said in the name of Shmuel: From where [do we know] that a niddah counts for herself? As it is stated “and she shall count for herself” — for herself.
Self-attestation of the count
Chazal establish that she is believed regarding her own count of clean days without requiring witnesses.