What is the ruling if one performs tayammum, begins a supererogatory (nafl) prayer, and then sees water?
Chapter on Tayammum
Al-Mughni
Book of Purification
Primary text
If the individual intended to complete a specific number of units (rak'ahs) in the nafl prayer upon seeing water, they must complete those intended units. If no specific number was intended, they are permitted to complete a maximum of two rak'ahs, according to the apparent position of the madhhab, as this is the minimum length of a prayer.
Supporting text
The view held strongly by the Sheikh is that if the initial prayer is not invalidated by seeing the water, the person may initiate another prayer. This is because seeing the water did not nullify the tayammum itself; if the tayammum were nullified, the ongoing prayer would have been voided, and subsequent actions would not affect the past. This situation resembles the case where water is seen but a barrier (like a predator, siba') remains between the person and the water, allowing them to pray as much as they wish, as if they had never seen the water.