It is narrated from Sayyidah Umm Salamah (may Allah be pleased with her) that in the time of the Messenger of Allah (peace and blessings be upon him) a woman used to bleed continuously, so Sayyidah Umm Salamah (may Allah be pleased with her) asked the Prophet (peace and blessings be upon him) about her, and the Prophet (peace and blessings be upon him) said: "Let her look at the number of days and nights in which she used to menstruate before this (istihadah) befell her, and their extent in the month, and let her leave the prayer for that much. Then when this (period) has passed and the time of prayer comes, let her take a ritual bath (ghusl) and pray."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20222