Tayammum is performed by muslims when

Tayammum is performed by Muslims when access to water is very difficult or when it is impossible to use water for purification due to some reasons, such as unavailability of water, lack of time, or the presence of an illness that makes it impossible to use water. Tayammum is a dry ablution, which is a substitute for the traditional washing with water (wudu or ghusl). It involves striking the hands on clean, dry earth or sand and then using the resulting dust to wipe the face and the arms. Tayammum is a way for Muslims to maintain cleanliness before performing prayers, even when water is not available.