Reference for Niddah 106:43

אמר רב חסדא הכי קאמר

— R. Hisda replied: It is this that was meant: IF A WOMAN OBSERVED A DISCHARGE OF BLOOD ON THE ELEVENTH DAY AT TWILIGHT a time which is THE BEGINNING OF A MENSTRUATION PERIOD AND THE END OF A <i>ZIBAH</i> PERIOD, or on the seventh day of her menstruation when it is THE END OF A MENSTRUATION PERIOD AND THE BEGINNING OF A <i>ZIBAH</i> PERIOD.

Jastrow