How is menstruation determined when purity inside the flow prevents considering subsequent flow as menses before recurrence?
Chapter on Menstruation
Al-Mughni
Book of Purification
Primary text
If the intervening purity prevents subsequent flow from being counted as menses before recurrence, and the flow aligns with her usual habit, she must combine the subsequent flow with the first instance until the minimum duration of menses is completed. For example, if she bleeds one day and is pure one day, the third instance of blood is joined with the first. If she sees less than the minimum menses, then is pure for thirteen days, and then sees blood similar to the first, and the minimum purity is thirteen days, the second flow is blood of corruption because one instance is disqualified by the minimum purity period, and the two instances combined do not form two menses due to the intervening minimum purity period.
Supporting text
If the minimum purity is fifteen days, the second instance is joined to the first to form a single menses if their combined span does not exceed fifteen days. If each instance of blood meets the minimum requirement for menses, they are counted as two separate menses if the minimum purity is thirteen days. If the minimum purity is fifteen days, the second is joined to the first to form one menses, provided the span between the ends of the flows is not more than fifteen days. If the span is fifteen days, they cannot both be counted as menses; one is menses and the other is Istihadah. This principle applies analogously to post-natal bleeding (Nifas).