Fiqh-us-Sunnah Volume 1, Purification and Prayer, Fiqh 1.071A.
Section : They cannot fast, menstruating women.
If women fast, their fasting will be considered null and void. If they fast during the month of Ramadan, they will still have to make those days of fasting up later on. Mu’azhah said, “I asked ‘Aishah, ‘Why must we make up the fasts missed due to our menstruation, and not the prayers?’ She said, ‘That was what the Messenger of Allah told us to do. We were ordered to make up the fasts, and we were ordered not to make up the prayers.” (Related by “the group.”)