Praise be to Allaah.
Whoever cannot do wudoo’ with water has to do tayammum and
pray with tayammum. Tayammum (dry ablution) means striking the palms of
one's hands on the ground then wiping the face and palms.
It should be noted that Tayammum is prescribed by Allaah
when there is no water or when a person is unable to use it because of
sickness and the like. Allaah says (interpretation of the meaning):
“O you who believe! Approach not As‑Salaah (the prayer) when
you are in a drunken state until you know (the meaning) of what you utter,
nor when you are in a state of Janaaba (i.e. in a state of sexual impurity
and have not yet taken a bath), except when travelling on the road (without
enough water, or just passing through a mosque), till you wash your whole
body. And if you are ill, or on a journey, or one of you comes after
answering the call of nature, or you have been in contact with women (by
sexual relations) and you find no water, perform Tayammum with clean earth
and rub therewith your faces and hands (Tayammum). Truly, Allaah is Ever
Oft‑Pardoning, Oft‑Forgiving” [al-Nisa’ 4:43]
Fataawa al-Lajnah al-Daa’imah. See Fataawa Islamiyyah, part 1, p. 216