Although it is permissible to shower to cool down and rinse the mouth to quench thirst while fasting, if water is swallowed while showering or rinsing the mouth, and the fasting individual remembers that they are fasting, the fast will be deemed invalid. In such a case, the fasting individual must perform qaza for that day’s fast.
Moreover, swallowing the film of moisture that remains in the mouth after rinsing the mouth, is permissible and does not invalidate the fast, provided that the individual spits once or twice after rinsing their mouth in order to get rid of the water properly. After spitting once or twice, most of the remaining water is expelled and the film of moisture that remains even after spitting can be swallowed without any sin.
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دلائل:
الھندیة: (کتاب الصوم، 199/1، ط: دار الفکر)
وعن أبي حنيفة - رحمه الله تعالى - أنه يكره للصائم المضمضة والاستنشاق بغير وضوء وكره الاغتسال وصب الماء على الرأس والاستنقاع في الماء والتلفف بالثوب المبلول وقال أبو يوسف: لا يكره، وهو الأظهر كذا في محيط السرخسي.
Allah the Almighty knows best.
Darul ifta Al-Ikhlas, Karachi.