Is it permissible to perform tayammum when water is unavailable if there is cold water in the refrigerator and using it is difficult? And is it obligatory to store water for ablution if its unavailability is a daily recurring issue? And is a Muslim sinful if they do not do so, or if they perform tayammum out of forgetfulness or finding it bothersome?

Wudu' (ablution) is a condition for the validity of prayer. It is not permissible to resort to tayammum (dry ablution) except when water is unavailable, or when there is a certain or probable fear of harm from using it, after attempting to avoid the harm through means such as heating.

Accordingly, it is not permissible to perform tayammum when cold water is available in the refrigerator. Rather, one must perform wudu' with it or heat it. The hardship resulting from the coldness of the water is not considered unless it reaches the degree of harm.

As for storing water for purification, it becomes obligatory after the entry of the prayer time for someone who knows they will not find other water, and one who deliberately fails to store it is sinful. Their prayer performed with tayammum is valid if they searched for water and did not find it. Before the entry of the prayer time, storing water is not obligatory, but it is advisable for someone who anticipates water unavailability.