Is the obligation to perform a missed prayer established immediately upon entering the time for that prayer?
Chapter on Prayer Times
Al-Mughni
Book of Prayer
Primary text
The obligation to perform a missed prayer is established immediately upon entering the prayer time. If a person enters a portion of the time and then experiences an incapacitating condition, such as unconsciousness or menstruation, they are obligated to make up the prayer when able. This is because the prayer became obligatory upon them, and therefore its missed performance requires subsequent fulfillment, analogous to a prayer that was fully within the prescribed time. This view is supported by the general principle that a debt owed must be paid when due.
Supporting text
Al-Shafi'i and Ishaq maintain that the obligation (Istiqrar) is not established unless a duration sufficient to perform the prayer has passed. Abu Abdullah ibn Battah favored this view, arguing that if the time duration necessary for prayer was not experienced, no makeup obligation arises, similar to a situation where the impediment existed before the time began.