Musannaf Ibn Abi Shayba 1:1361

Waki' narrated to us, he said: Al-A'mash narrated to us, from Al-Minhal, from Sa'id ibn Jubayr, who said: I was with Ibn Abbas when a woman came with a letter. He read it, and behold, in it was: "I am a woman suffering from *istihada* (prolonged non-menstrual vaginal bleeding), and Ali said: 'She should perform *ghusl* (full ablution) for every prayer.'" Ibn Abbas said: "I find nothing for her except what Ali said."