What is the required procedure for a woman accustomed to a cessation period long enough for purification and prayer, when bleeding occurs?
Chapter on Menstruation
Al-Mughni
Book of Purification
Primary text
A woman accustomed to a cessation period long enough for purification and prayer should not immediately commence prayer while bleeding. She must wait until the bleeding stops, unless she fears the exit of the prescribed prayer time. In that case, she should perform ablution and pray. If she begins the prayer at the end of the time limit using this purity, and the bleeding stops during the prayer, her purification is invalidated. Since she was able to perform the prayer with non-obligatory purity (i.e., purity that was not strictly necessary due to the ongoing flow), her prayer using the alternate purity is invalid, similar to a non-mustahadah (woman with non-menstrual, continuous flow).
Supporting text
If the duration of cessation is variable, sometimes ample and sometimes not, she is treated as one whose cessation time is variable. However, if she knows the cessation during the current period will not be ample, her situation is clear. There is an alternative view that if she begins prayer and then the bleeding stops, her prayer is not invalidated, because she commenced with a certain purity, and the cessation might have been long enough (a matter of doubt, which does not overturn certainty). But if the cessation proves continuous, it becomes clear that the purity was invalidating, thereby invalidating both the purification and the prayer.