It is narrated from Sayyidah Fatimah bint Qays (may Allah be pleased with her), who was from Banu Asad of Quraysh, that she came to the Messenger of Allah (peace and blessings be upon him) and mentioned to him that she suffered from istihadah (prolonged bleeding). Fatimah stated that he (peace and blessings be upon him) said: "That is only a vein. When the menstruation comes, leave the prayer, and when it ends, wash off the traces of the blood and pray."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 21268