Questioner: A man is praying having performed dry-ablution [tayammum], and then during the prayer water becomes available, what should he do?
Al-Albaani: He cuts off his prayer, and uses the water [to perform wudoo].
Questioner: Okay, what if he has given the tasleem and then water becomes available? After he had given the tasleem? [i.e., after he has completed his prayer?]
Al-Albaani: Khalaas …
Al-Albaani: Thus is the case judged concerning which you both did inquire. His prayer is correct, but he must perform wudoo for the next one.
Al-Hudaa wan-Noor, 438.