If it was done mistakenly, the fasting is valid.

Sheikh, what happens to the fast of a muslim who mistakenly swallows water or toothpaste while fasting (when cleaning the teeth).

Fasting | October 7, 2017 12:02 am
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My wife has missed fasts during ramadan because of pregnancy.. She wants to keep the fasts of ashura. She was wondering can she combine the intention of ashura fast/missed ramadan for one fast. Or do missed ramadan fasts have to be done separately.. can she combine the intentions.

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I was thinking to start fasting In sha Allah one day fast one day off starting from Thursday. Is it permissible in Islam?