It is narrated from Sayyida Umm Salama (may Allah be pleased with her) that during the time of the Prophet (peace be upon him), a woman used to experience excessive bleeding of istihada. Umm Salama (may Allah be pleased with her) asked the Messenger of Allah (peace be upon him) about her issue, so he (peace be upon him) said: "Before this illness began, the woman should see how many days her menstruation used to last. She should count those days and refrain from prayer. When those days have passed, she should perform ghusl, then tie a cloth (to prevent the blood), and perform prayer."
The author has mentioned here the fatwa of Imam Shafi’i (may Allah have mercy on him): If a woman used to have known days of menstruation and then developed the illness of istihada, and the blood continues to flow and does not stop, then she should estimate her prayers and fasts based on her previous menstruation days. She should not decrease or increase even by an hour, but should perform ghusl and pray. However, she should make fresh wudu for every prayer. She may also perform nafl prayers, but for the next obligatory prayer, she should make fresh wudu. If the istihada sometimes stops, then when she sees thin, yellowish blood, that is istihada, and when the blood is mostly red, that is menstruation. Upon its end, she should perform ghusl and make fresh wudu for every prayer, and continue to pray during istihada.
Both types of women should not decrease or increase even by an hour in the name of precaution, but should act upon the rulings of prayer, fasting, and marital relations. As for the woman who used to perform ghusl for every prayer and sit in a tub, the Messenger of Allah (peace be upon him) did not command her to perform ghusl; she used to do it herself.
Imam Shafi’i (may Allah have mercy on him) said: As for the woman who has been suffering from istihada from the very beginning and her menstruation days are not known, and the blood of istihada does not stop, then we will not estimate her days based on her mother or sisters (i.e., count as many days as they have menstruation). She has a separate ruling: the shortest period of menstruation for which a menstruating woman leaves prayer is one day. Such a woman will leave prayer for one day, then she will pray.
Hadith Referenceالسنن المأثورة / باب ما جاء في الصلاة على الراحلة / 136
Hadith Takhrijسنن ابی داؤد، الطهارة، باب في المرأة تستحاض... الخ، رقم: 274، وقال الالبانی: صحيح؛ سنن نسائی، الطهارة، باب ذكر الاغتسال من الحيض، رقم: 208؛ مسند احمد: 123/44، رقم: 26510، وقال الارنوؤط: حديث صحيح رجاله ثقات