It is narrated from Sayyidah Aishah (may Allah be pleased with her) that Fatimah bint Abi Hubaysh (may Allah be pleased with her) came to the Messenger of Allah (peace and blessings be upon him) and said: I suffer from istihadah (prolonged bleeding) and do not become pure; shall I then leave the prayer? 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, wash off the traces of the blood and pray."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 21262