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 Prophet (peace and blessings be upon him) and said: O Messenger of Allah, I am a woman with istihadah and I do not become pure; should I leave prayer? He (peace and blessings be upon him) said: "No, that is a vein, not menstruation. When your menstruation comes, leave prayer; and when it goes, wash the blood from your body, then pray." The narrator said: My father said: Then perform ablution for every prayer until that time (of menstruation) comes.
Urwah's statement in it is sahih (authentic).
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1624