Imâm Muhammad bin Sâlih bin `Uthaymeen (d. 1421)
Source: Majmoo`-ul-Fatâwâ (19/107-108)
Question: A woman did not know if she still had her menstrual cycle or not. She was clean in the morning. Is her fast valid, bearing in mind that she did not know if she was clean?
Answer: The mentioned woman’s fast is invalid. She has to make up for that day, because she had to assume that she still had her menstrual cycle. To start the fast without knowing if she is clean or not is like entering into worship with doubts as to its validity. Therefore, it is invalid.