I say, with Allah's guidance: He must imitate those who pray obligatorily, so he should bow and prostrate, then repeat the prayer: like fasting, and this is the ruling given. Similarly, if someone is confined in a filthy place and cannot find water or bring out purifying dust, the same ruling applies; as narrated by Ibn Umar  the Prophet  said: "No prayer is accepted without purification, and no charity from treachery," in Sahih Muslim 1: 204, and Sahih Bukhari 1: 63. See: Al-Durr Al-Mukhtar and Radd Al-Muhtar 252-253, and Allah knows best.