Must a person performing prayer without Wudu (ablution) exit the prayer if they find water or dust during the prayer?
Chapter on Tayammum
Al-Mughni
Book of Purification
Primary text
If a person performs the prayer in a state without Wudu and without Tayammum (dry ablution), and subsequently finds water or dust during the prayer, the prayer must be terminated in every case. This is because the prayer was performed without ritual purity, which is a necessary condition. This ruling is based on the principle that if purity is a prerequisite, and the prerequisite is later found, the ritual invalidates the act unless one holds the view that redoing the prayer is not obligatory upon finding water later.
Supporting text
It is plausible that the person should exit the prayer in a manner similar to how one exits the prayer when required to perform Tayammum if water is found later, provided we hold the view that redoing the prayer is not obligatory. This scenario is likened to a person who prays naked due to inability to find covering, and then finds covering nearby during the prayer; in such cases where the prayer must be repeated, exiting upon removal of the impediment (finding water/dust) and restarting the prayer is required. If redoing the prayer is deemed unnecessary, then the situation resembles that of a person performing Tayammum who finds water, as previously discussed regarding that matter.