Musannaf Ibn Abi Shayba 10:1013

Abu Bakr narrated to us, he said: Ya'la ibn 'Ubayd narrated to us, from Yahya ibn Sa'id, from 'Urwah, from 'Aishah, she said: "The *aqra'* (menstrual cycles/periods of purity) are only the periods of purity."