Tayammum is a ritual purification in Islam performed with clean earth or dust, which is an alternative to wudu (ablution) or ghusl (full-body washing) when water is not available or its use is prohibited, or when a person is unable to use water due to illness or disability. So, performing Tayammum is justified when a person is unable to use water due to a valid reason such as the unavailability of water, sickness, or other reasons where the use of water is not possible.