Yahya related to me from Malik from Hisham ibn Urwa from hisfather that A'isha, the wife of the Prophet, may Allah bless him andgrant him peace, said, "Fatima bint Abu Hubaysh said, 'Messenger ofAllah, I never become pure - am I permitted to pray?' The Messenger ofAllah, may Allah bless him and grant him peace, said, 'That is a vein,not menstruation. So when your period approaches, leave off from theprayer, and when its grip leaves, wash the blood from yourself andpray.' "
USC-MSA web (English) reference: Book 2, Hadith 106
Arabic reference: Book 2, Hadith 135