She bathed and fasted before completing her purification from menstruation

Question:
May Allah reward you with good and do you good. A sister of ours, whose name is Shaim Muhammad al-Hazimi, asks a number of questions and says: What is the ruling on a woman who bathed and fasted while she had not yet become pure? Is her fast for that day valid?

Answer:
Her fast is not valid. If she fasted before the blood stopped, and the dawn broke upon her, and she fasted while the blood was still present, then her fast is not valid. This is because a portion of that day passed while she was menstruating. Bathing does not make fasting permissible for her unless the bathing took place after the blood had stopped. Therefore, the woman should not rush; she must wait until she sees complete purity and cleanliness. If this happens before the dawn, she should fast, even if she has not bathed. If she delays the bath until after the dawn, there is no harm in that. Yes.