Reference for Zevachim 75:2

אלא מעתה (ויקרא יב, ה) וטמאה שבועים שבעים אהני קרא ואהני מסורת ארבעים ותרין בעיא למיתב

If so, [when Scripture states] Then she shall be unclean two weeks [shebu'ayim] shib'im [seventy] [is actually written], then [argue,] the mikra and the masoreth are both effective, and so she should have to spend forty-two days [in uncleanness]? - There it is different, because it is written, as in her menstrual state. <br>

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