Musnad Ahmad 1153:2236

Ali ibn Hashim ibn al-Barid narrated to us, in the year seventy-seven, from al-A’mash, from Habib, from ‘Urwah, from ‘Aishah, who said: Fatima bint Abi Hubaysh came to the Prophet, may Allah bless him and grant him peace, and said: "O Messenger of Allah, I have experienced *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."