Musannaf Ibn Abi Shayba 1:1344

Abu Bakr narrated to us, he said: Waki' narrated to us, from Hisham ibn 'Urwah, from his father, from 'Aishah, she said: Fatima bint Abi Hubaysh came to the Prophet, may Allah bless him and grant him peace, and said: "O Messenger of Allah, I am a woman who suffers from *istihada* (prolonged non-menstrual bleeding), so I do not become pure. Should I abandon the prayer?" He said: "No, that is only a vein, and it is not the menstruation. So when the menstruation approaches, abandon the prayer; and when it departs, wash the blood from yourself and pray."