Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.066D.

In addition to the presence of water, everything that nullifies the ablution nullifies tayammum. If a person prays after performing tayammum and then finds water, he does not need to repeat his prayer even if there is time left to do so. Abu Sa’eed al-Khudri said, “Two men went out on a journey. The time of prayer came and, as they had no water, they performed tayammum. Then they found some water during the time of the same prayer. One of them repeated his prayer with ablution and the other did not. When they saw the Messenger of Allah, they asked him about the proper procedure in such a case. He said to the one who did not repeat his prayer, ‘You have acted according to the sunnah and your prayer is sufficient for you.’ He said to the other, ‘You will get a double reward.”‘ (Related by Abu Dawud and anNasa’i.) If one comes across water before he prays or finishes his prayer with tayammum, his prayer becomes null and void, for he must make ablution with water. If a person is not clean because of sex or a wet dream, or a woman is menstruating, and they pray after performing tayammum, they need not repeat their prayer after finding water, but they must perform ghusl with water when they can. ‘Umar said, “The Prophet led the people in prayer, and afterwards saw a man who had not prayed. He said, ‘Why didn’t you pray with us?’ The man replied, ‘I was sexually unclean and there was no water.’ He told him, ‘Use the soil, and it will be enough.’ ‘Imran then mentioned that they later found water. The Prophet, upon whom be peace, brought a bowl of water for the man and told him to perform ghusl. (Related by al-Bukhari.)