I say, and with Allah's success: It is recommended for one who hopes to find water, and is confident in his expectation of finding it, to delay his prayer until the end of the time; to perform it in the best possible manner. However, if he prays with tayammum at the beginning of the time and then finds water while time remains, he is not required to repeat the prayer. As for one who does not hope to find water, he should not delay the prayer beyond the beginning of the time; because the benefit of waiting is the possibility of finding water, so he performs it with both forms of purification, which are not present here. Ali (may Allah be pleased with him) said: "If a man becomes junub while traveling, he should wait until the last part of the time. If he does not find water, he should perform tayammum and pray," in Sunan al-Bayhaqi 1: 233. And Abu Sa'id (may Allah be pleased with him) said: "Two men went on a journey, and the time for prayer came while there was no water. They performed tayammum with pure soil, prayed, and then found water during the time. One of them repeated the prayer and the ablution, while the other did not. Then they came to the Messenger of Allah (peace be upon him) and mentioned this to him. He said to the one who did not repeat: 'You have followed the Sunnah and your prayer is valid.' And he said to the one who performed ablution and repeated: 'You have a reward twice,'" in Al-Mustadrak 1: 286, which is authenticated, and in Sunan al-Darimi 1: 207, Sunan al-Bayhaqi al-Kabir 1: 231, Sunan Abi Dawood 1: 93, and Al-Mujtaba 1: 213. See: Sharh al-Wiqayah p. 112, and Al-Bahr al-Ra'iq 1: 163-164, and Allah knows best.