What should one do about tahaarah (purification) if one can find neither water nor dust?
Question

                What should a person do if he finds no water and no dust to purify himself with? Does he have to repeat his prayer after he finds either of them?
Praise be to Allah. 
          Ibn Hazm said: 
 
          Whoever is detained, whether he is traveling or not, 
          in such a state that he cannot find dust or water, or he is being crucified, 
          and the time for prayer comes, then he should pray as he is, and his 
          prayer will be complete and does not have to be repeated, whether he 
          finds water within the time of the prayer or after the time is over. 
 
          The proof of that is the aayahs (interpretation of the 
          meaning): 
 
          “So 
          keep your duty to Allaah and fear Him as much as you can”
          [al-Taghaabun 64:16]
 
          “Allaah burdens not a person beyond his scope”[al-Baqarah 
          2:286]
 
          And the Messenger of Allaah 
          
          (peace and blessings of Allaah be upon him) said: “If I command you 
          to do a thing, then do as much of it as you can.” 
 
          And Allaah says (interpretation of the meaning): 
 
          “… while He has explained to you in detail what is 
          forbidden to you, except under compulsion of necessity?”[al-An’aam 6:119]
 
           From these texts we see  that we are not obliged 
          to do more than we are able to, and that if we are unable to do something, 
          we are relieved of the obligation. We learn that Allaah has forbidden 
          us to neglect wudoo’ or tayammum for prayer, unless we are forced “under 
          compulsion of necessity”. The one who is deprived of water and dust 
          is forced by necessity to do something which is forbidden, i.e., to 
          forgo purifying himself with water or dust; so the prohibition does 
          not apply in his case. But he is able to pray, fulfilling all the rulings 
          on prayer and believing it to be obligatory, so it remains for him to 
          do that which he is able to do. If he prays as we have mentioned, then 
          he has prayed as Allaah has commanded him to do, and whoever prays as 
          Allaah has commanded him to do is not to blame. And hastening to pray 
          at the beginning of the time for prayer is preferable, because of what 
          we have stated above. 
 
          Abu Haneefah, Sufyaan al-Thawri and al-Oozaa’i said, 
          concerning one who is in this situation, that he should not pray until 
          he finds water, when he finds it.