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 abandon the prayer?" The Messenger of Allah (peace and blessings be upon him) said: "That is a vein; it is not menstruation. When it comes, abandon the prayer, and when its (the menstruation's) duration has passed, wash the blood off yourself and pray."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1565