It is narrated from Sayyidah Aishah (may Allah be pleased with her) that Fatimah bint Abi Hubaysh (may Allah be pleased with her) said to the Messenger of Allah (peace and blessings be upon him): "O Messenger of Allah, I do not become pure; shall I leave the prayer?" The Messenger of Allah (peace and blessings be upon him) said: "That is from a vein; it is not menstruation. When menstruation comes, leave the prayer, and when its duration has passed, wash the blood off yourself and pray."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1535