It is narrated from Urwah (may Allah have mercy on him) that Fatimah bint Abi Hubaysh (may Allah be pleased with her) had prolonged bleeding (istihadah), and the Prophet (peace and blessings be upon him) said to her: "The blood of menstruation is black and recognisable. When it is that, refrain from the prayer, and when it is the other, perform ablution and pray, for it is from a vein."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1551