If the menstruating woman becomes pure before Fajr and performs ghusl only after it, is her fast valid?

Original answer — Shaykh ‘Abd al-‘Aziz ibn Baz

Question

«إذا طهرت الحائض قبل الفجر واغتسلت فما الحكم؟»

English translation (unreviewed): If the menstruating woman becomes pure before Fajr and performs ghusl, what is the ruling?

From the text of the answer

«صومها صحيح إذا تيقنت الطهر قبل طلوع الفجر، المهم أن المرأة تتيقن أنها طهرت؛ لأن بعض النساء تظن أنها طهرت وهي لم تطهر، ولهذا كانت النساء يأتين بالقطن لعائشة رضي الله عنها فيرينها إياه علامة على الطهر، فتقول لهن: لا تعجلن حتى ترين القصة البيضاء.»

English translation (unreviewed): Her fast is valid if she was certain of purity before the dawn broke. What matters is that the woman be certain that she has become pure, because some women think they have become pure when they have not. For this reason, women used to bring cotton to 'A'ishah, may Allah be pleased with her, showing it to her as a sign of purity, and she would say to them: “Do not hurry until you see the white discharge.”

Evidence as the shaykh cited it

«ومثل المرأة الحائض من كان عليها جنابة فلم تغتسل إلا بعد طلوع الفجر فإنه لا حرج عليها وصومها صحيح، كما أن الرجل لو كان عليه جنابة ولم يغتسل منها إلا بعد طلوع الفجر وهو صائم فإنه لا حرج عليه في ذلك؛ لأنه ثبت عن النبي ﷺ أنه يدركه الفجر وهو جنب من أهله فيقوم ويغتسل بعد طلوع الفجر، والله أعلم [١] .»

English translation (unreviewed): Like the menstruating woman is one who had janabah and did not perform ghusl until after the dawn broke: there is no harm on her and her fast is valid, just as a man who was in a state of janabah and did not perform ghusl until after dawn while fasting, there is no harm on him in that; because it is established from the Prophet ﷺ that dawn would come upon him while he was junub from his wife, so he would get up and perform ghusl after dawn. And Allah knows best [1].