If one who performed Tayammum for oneself finds water, is it permissible for them to leave the prayer to use the water?
Chapter on Tayammum
Al-Mughni
Book of Purification
Primary text
If one has entered the prayer via Tayammum and subsequently finds water, there are two viewpoints regarding permission to leave. The first view permits leaving the prayer, as the person has commenced an act pertaining to a substitute (Tayammum), and thus they have the choice between returning to the original act (Wudu/Ghusl) or completing the act already commenced. This is analogous to one who begins a fast of expiation (Kaffarah) when the option of freeing a slave becomes available.
Supporting text
The second viewpoint prohibits leaving the prayer, based on the principle that what does not necessitate leaving the prayer (i.e., finding water does not obligate leaving) does not permit leaving it, similar to all other ancillary matters. The scholars associated with the Shafi'i school hold these two equivalent viewpoints.