Hadrat Abu Said al-Khudri (Radi Allaahu Ta’ala Anhu) narrated that from amongst the Companions of the Messenger of ALLAH two persons went out on journey, and when the time for Salaah approached, having no water with them, they performed ablution with clean earth and then performed Salaah. Immediately after that, they found water and one of them repeated the Salaah after performing ablution with water, but the other did not.
When they came to the Messenger of ALLAH sometime afterwards they mentioned what they had done. The Messenger of ALLAH (Sallallahu Alaihi Wa Sallam) said to the one who had not repeated the Salaah, “You have observed the sunnah and your Salaah was enough for you”, To the other who had repeated the Salaah after performing ablution, he said: “You will have a two fold reward for your second Salaah became supererogatory (NAFL), and ALLAH does not leave good deeds unrewarded”.
The Shariah, therefore, lays down that it is not necessary to repeat a Salaah said with tayammum. [Sunan Abu Dawud, Musnad Darime, Ma’rif -ul- Hadis]