It is narrated from Sayyidah Aishah (may Allah be pleased with her) that Fatimah bint Abi Hubaysh (may Allah be pleased with her) said: O Messenger of Allah! I do not become pure; shall I then leave the prayer? The Messenger of Allah (peace and blessings be upon him) said to her: "That is only a vein, not menstruation. When the menstruation comes, leave the prayer, and when it ends, wash off the blood, take a ritual bath (ghusl) and pray."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 21258