Sahih Muslim | Book 3 | Hadith Number 659
Narrated by 'Aisha
'Aisha, the wife of the Apostle (may peace be upon him), said: Umm Habiba b. Jahsh who was the spouse of Abd al- Rahman b. Auf made a complaint to the Messenger of Allah (may peace be upon him) 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.