Musnad Ahmad 1153:136

Ali ibn Hashim narrated to us, al-A’mash narrated to us, from Habib, from ‘Urwah, from ‘Aishah, who said: Fatima bint Abi Hubaysh came to the Prophet, peace and blessings of Allah be upon him, and said: "I have been experiencing *istihada* (prolonged non-menstrual bleeding)." He said: "Leave the prayer during the days of your menstruation, then perform *ghusl* (full ablution), and perform *wudu* (ablution) for every prayer, even if it drips onto the mat."