Tayammum on the Basis of an Imagined Fear of Illness
Question
Although there is no repeated experience [that ghusl causes harm], yet whenever [a person] performs ghusl something or other certainly happens — what should be done then? Sometimes, because of weakness of heart and weakness of constitution, a waswasa (obsessive thought) arises that perhaps one will fall ill from ghusl and wudu — [may one] perform tayammum on that basis? Or [a person] felt sluggish and performed tayammum lest he become unwell.
Answer
Mere lack of resolve, laziness and imagination count for nothing. Yes, if there is repeated experience that performing ghusl or wudu brings on illness or increases illness, then tayammum is permitted. Then, when this excuse no longer remains, it is necessary to perform ghusl so that the next prayer is performed with ghusl.
Source: Fatawa Mahmudiyya
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