What is the minimum period of purity (Tuhur) between two menstrual cycles?
Chapter on Menstruation
Al-Mughni
Book of Purification
Primary text
The minimum duration of purity between two menstrual periods is thirteen days. This ruling is supported by the view of Ahmad, which holds that the waiting period (Iddah) can be completed within a single month if evidence supports it. This is based on the saying attributed to Ali, who, when presented with a woman claiming three menstruations in one month, endorsed the testimony of reliable, pious women from her inner circle, implying that the minimum purity period must allow for this scenario (at least thirteen days). This opinion, narrated by Imam Ahmad with a sound chain, is considered authoritative because it is a statement by a Companion that spread without known contradiction, which necessitates a minimum purity period that aligns with the maximum duration of menstruation being fifteen days (if minimum purity is thirteen days). The minimum duration of a menstrual period is one day and one night.
Supporting text
Malik, Al-Thawri, Al-Shafi'i, and Abu Hanifa hold that the minimum period of purity is fifteen days. Ishaq rejects the timing set by some at fifteen days. Abu Bakr based his calculation on the maximum duration of menstruation: if the maximum menstruation is fifteen days, minimum purity is fifteen days; if the maximum is seventeen days, minimum purity is thirteen days. This latter view presupposes that a woman's lunar month does not exceed thirty days, within which menstruation and purity combine. If the month exceeds this, it is conceivable for her menstruation to be seventeen days and her purity fifteen days or more.