A woman fasted while she is not sure if she is pure of her menstrual period. The next day she was sure that she is pure. What is the ruling on fasting the day she doubted her purity?.
Her fast is not valid and she has to make up that day, because the basic principle is that the menses was still ongoing, and her beginning to fast when she was not sure that she had become pure was embarking upon an act of worship when she was not sure that the conditions of its validity were met, and this means that it is not valid. End quote.
Shaykh Muhammad ibn ‘Uthaymeen.