Muwatta Malik 134

Yahya related to me from Malik from Hisham ibn Urwa from his father that A'isha, the wife of the Prophet, may Allah bless him and grant him peace, said, "Fatima bint Abu Hubaysh said, 'Messenger of Allah, I never become pure - am I permitted to pray?' The Messenger of Allah, may Allah bless him and grant him peace, said, 'That is a vein, not menstruation. So when your period approaches, leave off from the prayer, and when its grip leaves, wash the blood from yourself and pray