The ruling on a menstruating woman fasting when she is uncertain about her purity

Question:
May Allah reward you, Honorable Shaykh! A woman says that when she was young, in one of the Ramadans, her mother ordered her to fast at the end of her period, and she was not sure at that time whether she had become pure or not. What is obligatory for her?

Answer:
If she was menstruating, she does not fast until the blood stops and she sees the purity and the white discharge. If she fasted while menstruating, or fasted while not knowing whether the blood had stopped or not, she must repeat the fast, because she does not clear her obligation regarding it except with certainty, and this is a fast in which she is uncertain. This is if the uncertainty occurred at the time of fasting. As for if the day she fasted she would become pure, but doubts came to her afterwards, she should not pay attention to them, because doubt after the act of worship has no effect. The doubt that has an effect is the doubt that comes during the act of worship, and it is not from the category of obsessive doubts.