Narrated Abu Sa'id al-Khudri (RAA)

Two men set out on a journey and when the time of Salat (prayer) came they had no water. They performed Tayammum with clean earth and prayed. Later on they found water within the time of the prayer. One of them repeated his prayer with ablution but the other did not repeat. Then they came to Allah's Messenger (ﷺ), and related the matter to him. Addressing himself to the one who did not repeat, he said, "You followed the Sunnah and your (first) prayer was enough for you."[9] He said to the one who performed ablution and repeated: "For you there is a double reward". [Reported by Abu Dawud and An-Nasa'i]