It is narrated from Sayyidah Aishah (may Allah be pleased with her) that Umm Habibah (may Allah be pleased with her) asked the Messenger of Allah (peace and blessings be upon him) about the blood. Sayyidah Aishah (may Allah be pleased with her) says: I saw her tub filled with blood. The Messenger of Allah (peace and blessings be upon him) said to her: "Wait for as long as your menstruation used to hold you back, then take a ritual bath."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1571