Reference for Shabbat 269:12

א"ל אביי דורות הראשונים יוכיחו שאין אמו טמאה לידה ונימול לשמנה

For it was stated: Rabbah said: We suspect that it may be a suppressed foreskin;<span class="x" onmousemove="('comment',' It is only because of this doubt that some drops of blood must be made to flow. ');">¹¹</span>

Jastrow