Dry Purification (Tayammum)
(If one needs to pray, but is in the state of ritual impurity, and cannot find water) either he cannot find it physically, by seeking it and not locating it with him or with his traveling companions, or in the allotted distance in which he is obligated to seek it; or he cannot find the water figuratively, by being in need of drinking the existent water; (or) he found the water however (he) fears that using it will cause his death or harm of one of his organs, or he fears that he (will be harmed upon using it,) by the water causing his sickness to prolong, then (one performs dry purification.)
It is a condition for the validity of dry purification (tayammum) that the (Tayammum is performed after the time of prayer, per se, sets in) i.e. after the time of the act of obedience which he wants to perform, like prayer and circumambulating the Ka^bah, (and) the tayammum must be (after the unexempted najas-filth is removed.) from the body. If one performed tayammum with the presence of najas-filth on his body, his tayammum is invalid. This is if he had water to remove the najas-filth, if not then it was said that his judgment is like the judgment of the one who has lost the ability to perform both purifications (faqid attahurayin).
(Tayammum is performed by using soil,) hence it is not valid to perform tayammum with other than soil like stone, (which is unmixed) Furthermore, the soil must be unmixed, free from ashes and the like; (and is purifying and dusty.) Tayammum is invalid with soil contaminated with najas-filth, like urine.
Tayammum is performed (on the face) by wiping it (and) wiping (the hands and forearms) along with the elbows (while preserving this order) hence, it is a must to observe wiping the face first i.e. before wiping the hands and forearms, the least of which would be (by striking the soil twice) i.e. transferring of the soil must be done at least twice, hence, one strike is insufficient, however, one can strike more than twice. It is a condition of validity of tayammum that the one performing tayammum strikes the soil (and have the intention of making the obligation of praying permissible for one], the intention should be established (while transferring the dust of the soil from the ground) i.e. transferring the soil to the organ subject of tayammum (and wiping the beginning) the first part (of the face).