It is narrated from Abu Salamah (may Allah have mercy on him) that Umm Abi Bakr (may Allah be pleased with her) narrated to him that Aishah (may Allah be pleased with her) informed her that the Messenger of Allah (peace and blessings be upon him) said, concerning the woman who has doubts after purity: "It is a vein," or he said: "They are veins."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1600