Reference for Ketubot 143:12

ומשמשתו נדה: היכי דמי אי דידע בה נפרוש אי דלא ידע נסמוך עילוה דא"ר חיננא בר כהנא אמר שמואל מנין לנדה שסופרת לעצמה שנאמר (ויקרא טו, כח) וספרה לה שבעת ימים לה לעצמה

Having intercourse with him during the period of her menstruation. What is the case? If he knew he could have abstained, if he did not know he should rely upon her, for R. Hinena b. Kahana stated in the name of Shmuel: From where do we know that a menstruant counts her own days of menstruation? As it is said, “Then she shall count for herself seven days” (Leviticus 15:28), “Lah” means for herself.

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