Reference for Niddah 126:55

(ויקרא טו, לא) והזרתם את בני ישראל מטומאתם מכאן א"ר ירמיה

— Both are required. For, if he had informed us of the former statement only, it might have been presumed that it applied only to the law relating to clean things but not to that relating to a woman's permissibility to her husband. Hence we were informed [of the latter statement]. And if [our information were to be derived] from the latter statement only it might have been presumed that near her settled period an additional 'onah is required, hence we were informed that only one 'onah is necessary.

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