Amatul Rahmân Omer:
O you who believe! when you get up for Prayer (perform the ablution, so as to) wash your faces and your hands upto the elbows, and pass your (wet) hands over your heads (for mashah), and (wash) your feet upto the ankles. And if you are under an obligation to perform a bath, then wash yourselves thoroughly clean by a bath. But if you have been taken ill or on a journey (while under obligation to take a bath) or one of you comes from the privy, or you have had (sexual) contact with women and you do not find water, then have recourse to clean pure dust and wipe your faces and your hands with it (as Tayammum). Allâh does not intend to impose any impediments on you, but He intends to purify you (internally and externally) and complete His blessings upon you so that you may give (Him) thanks.
Surah Al Maidah (The Table Spread with Food) Ayat 6