Musnad Ahmad 1261:0

Narrated to us by Muhammad ibn Ja’far, who said: Narrated to us by Shu’bah, from Hisham ibn ‘Urwah, from his father, from ‘Aishah, that Fatimah bint Abi Hubaysh came to the Prophet (ﷺ) and said: "O Messenger of Allah, I am a woman who experiences prolonged bleeding (istihadah), and I do not become pure. Should I abandon prayer?" He (ﷺ) said: "No, that is a vein, but abandon prayer for the number of days you used to menstruate, then perform ghusl and pray."