O you who believe! When you intend to offer As-Salat (the prayer), wash your faces ...

... and your hands (forearms) up to the elbows, rub (by passing wet hands over) your heads, and (wash) your feet up to ankles. If you are in a state of Janaba (i.e. had a sexual discharge), purify yourself (bathe your whole body). But if you are ill or on a journey or any of you comes from answering the call of nature, or you have been in contact with women (i.e. sexual intercourse) and you find no water, then perform Tayammum with clean earth and rub therewith your faces and hands. Allah does not want to place you in difficulty, but He wants to purify you, and to complete His Favour on you that you may be thankful.
Al Quran - Al Maada, Ayat-6