Whoever is deprived of water and soil must resemble the praying ones as an obligation; he should bow and prostrate if he finds a dry place, otherwise he should gesture while standing and then repeat the prayer, and this is what is fatwa'd. Likewise, the one unable to use water and soil due to illness has the same ruling; as narrated by Ibn Umar, may Allah be pleased with him, the Prophet (peace be upon him) said: "No prayer is accepted without purification, nor charity from ill-gotten gains," in Sahih Muslim 1: 204, and Sahih Bukhari 1: 63. Refer to: Al-Durr Al-Mukhtar and Rad Al-Muhtar 252-253, and Allah knows best.