Adi bin Thabit narrates from his grandfather (may Allah be pleased with him) that the Prophet (peace and blessings be upon him) said: "The woman with prolonged bleeding (mustahadah) should leave the prayer during the days of her menstruation, then take a ritual bath, perform ablution for every prayer, fast and pray."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الطهارة / 566