Mishkat al-Masabih #493

Abu Sa’id al-Khudri said: Two men went out on a journey, and when the time for prayer came, having no water with them, they performed ablution with good earth and then prayed. Immediately afterwards they found water, and one of them repeated the prayer with ablution, but the other did not. When they came to God’s messenger sometime afterwards they mentioned that, and he said to the one who did not repeat the prayer, "You have observed the sunna and your prayer was enough for you." To the one who had performed ablution and repeated the prayer he said, "You will have a twofold reward.” Abu Dawud and Darimi transmitted it, and Nasa’i transmitted something similar. He and Abu Dawud also transmitted it from ‘Ata’ b. Yasar in mursal form.