Permission for tayammum

O you who believe, when you rise for Salah, (prayer) wash your faces and your hands up to the elbows, and make Mash (wiping by hands) of your heads and (wash) your feet up to the ankles. If you are in a state of major impurity, cleanse yourselves well (by taking bath). If you are sick, or on a journey, or if one of you has come after relieving himself, or you have had sexual contact with women, and you find no water, then, go for some clean dust and wipe your faces and hands with it. Allah does not like to impose a problem on you; He, rather likes to cleanse you and to complete His favour upon you, so that you may be grateful.

Al maeda 6