It is narrated from Sayyidah Aishah (may Allah be pleased with her) that Fatimah bint Abi Hubaysh (may Allah be pleased with her) suffered from istihadah (prolonged bleeding), so she asked the Messenger of Allah (peace and blessings be upon him), and he (peace and blessings be upon him) said: "That is only a vein, not menstruation. When the menstruation comes, leave the prayer, and when it ends, take a ritual bath (ghusl) and pray."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 21257