The Shaykh (Al-Mufid), may Allah سبحانه وتعالى support him, then said: "If one begins a prayer with tayammum due to the absence of water, and then finds water after starting the prayer, if he has already recited the opening takbir of the prayer, he should not leave the prayer. However, if he has not yet recited the opening takbir, he should leave, perform wudu’, and restart the prayer, if Allah سبحانه وتعالى wills." The strongest evidence supporting this is that tayammum provides a legitimate excuse for entering prayer. Once the prayer has been started with tayammum, there is no obligation to leave it unless there is definitive evidence to invalidate the excuse. There is no such evidence here requiring one to leave a prayer initiated with tayammum after finding water.
Commentary #26