From Sahih Muslim (Hadith 760):
A'isha, the wife of the Apostle (ﷺ), said:Umm Habiba b. Jahsh who was the spouse of Abd al- Rahman b. Auf made a complaint to the Messenger of Allah (ﷺ) about blood (which flows beyond the menstrual period). He said to her: Remain away (from prayer) equal (to the length of time) that your menstruation holds you back. After this, bathe yourself. And she washed herself before every prayer
Reference: Muslim 760