It is narrated from Sayyidah Umm Salamah (may Allah be pleased with her) that he (peace and blessings be upon him) said about the woman with prolonged bleeding (istihadah): "She should look at the number of her days and nights during which she used to menstruate, then take a ritual bath and pray."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1584