Reference for Avodah Zarah 47:11

אימא ישראל היו משמרין אותה לאמא משעה שנוצרה וניחוש דילמא רבעוה לאמא דאמא כולי האי לא חיישינן

Rather we must say that the mother, too, was watched by Israelites since the time it was first formed. But let us suspect that they had bestiality with the mother's mother? This suspicious we are not.

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