Musannaf Ibn Abi Shaybah

Hazrat Ata says that two men became junub (in a state requiring ritual purification). Both performed tayammum and offered prayer. Later, both found water within the prayer time. One of them repeated the prayer, while the other was content with his previous prayer. This was mentioned to the Prophet (peace and blessings be upon him), and he said: The one who repeated the prayer will have double reward, and the other's prayer is also valid.