Reference for Megillah 39:12

איצטריך סלקא דעתך אמינא תיהוי כראיה ראשונה של זב וראיה ראשונה של זב איתקש לבעל קרי דכתיב (ויקרא טו, לב) זאת תורת הזב ואשר תצא ממנו שכבת זרע מה בעל קרי טובל ביום האי נמי ליטבול ביומיה

It is necessary, lest you think that she should be the same as the first observation of one who had a genital discharge, and the first observation of one who had a genital discharge has been compared to one who had a seminal issue, as it is written, “This is the law of him that has had a genital discharge (zav) and of him from whom the flow of seed has gone out.” (Leviticus 15:32) Just as one with a seminal issue immerses during the day, so too this one immerses on the same day.

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