Sayyiduna Abu Salamah and Ikrimah (may Allah have mercy on them both) report that Sayyidah Zaynab bint Umm Salamah (may Allah be pleased with her) used to observe i'tikaf with the Messenger of Allah (peace and blessings be upon him) while she was bleeding continuously. The Messenger of Allah (peace and blessings be upon him) ordered her: "She should perform a ritual bath (ghusl) for every prayer."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب الحيض / 1649