Tayammum

Tayammum (Arabic: تيمم‎‎) is the Islamic act of dry ablution using a purified sand or dust, which may be performed in place of ritual washing (wudu or ghusl) if no clean water is readily available or if one is suffering from moisture-induced skin inflammation or scaling.

"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. after a sexual discharge), purify yourselves (bathe your whole body). But if you are ill or on a journey, or any of you comes after 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 to you that you may be thankful."

Tayammum may be substituted for wudu or ghusl when access to water is restricted or impractical, namely:

Clarifying the meaning of the verse

4:43 O you who acknowledge, do not come near the contact prayer while you are drunk, until you know what you are saying. Nor if you have had intercourse, unless traveling, until you bathe. If you are ill, or traveling, or one of you come from the bathroom, or you had sexual contact with the women, and could not find water: then you shall seek clean soil and wipe your faces and hands. God is Pardoning, Forgiving.

The language of the phrase regarding tayammum (ablution without water) appears to be difficult to understand; but expressing it in symbolic logic makes it much easier to understand:

“...If you are ill, or traveling, or one of you come from the bathroom, or you had sexual contact with the women, and could not find water: then you shall seek clean soil and wipe your faces and hands”

Thus, in case of illness or travelling, even if there is water, one still does not need to wash.

Tayammum consists of the following steps:

The same conditions that invalidate wudu also invalidate tayammum. In addition, a person's tayammum is invalidated as and when water becomes available. Tayammum is very important when water is not near.

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Wikipedia