Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.064A.

Section : One cannot find water, or the amount one finds is insufficient for ablution.

‘Imran bin Husain said, “We were with the Messenger of Allah during a journey. When he led the people in prayer, one man stayed apart. He asked him, “What prevented you from praying?” He said, ‘I need a post-nocturnal bath and there is no water.’ He said, ‘Use the soil, for it is sufficient.”’ (Related by al-Bukhari and Muslim.)

Abu Zharr related that the Prophet, upon whom be peace, said, “The soil is a purifier for a Muslim, even if he does not find water for twenty years.” (Related by “the four.” At-Tirmizhi grades it hassan sahih.) But before one makes tayammum, he must look for water from any possible source. If he is sure water is not to be found or it is too far away, he does not have to look for it.