Musnad Abu Ya'la al-Mawsili 149:130

Ibrahim narrated to us, Hammad narrated to us, from Hisham ibn Urwah, from his father, from Aisha, that Fatimah bint Abi Hubaysh said: "O Messenger of Allah, I am a woman who experiences *istihada* (prolonged non-menstrual bleeding), so should I abandon the prayer?" The Messenger of Allah (peace be upon him) said: "That is only a vein, and it is not menstruation. So when the menstruation arrives, abandon the prayer; and when its flow departs, wash the blood from yourself, perform ablution, and pray."