What is the legal ruling regarding the marriage of a repentant adulteress whom God blessed with a husband who is unaware of her past, and after about ten years of marriage, she learned of the obligation to ensure the womb is clear by one menstrual cycle before marriage, and she does not remember if she had done so or not?

Some scholars hold that it is not obligatory to observe an 'iddah (definition) (waiting period) from zina (fornication/adultery). Ibn Qudamah said: "The woman who has committed zina is like the woman who has been carnally known by شبهة (sexual intercourse under misconception) regarding the 'iddah." There is another narration from Ahmad that she should observe istibra' (a waiting period to ensure no pregnancy) with one menstruation, and this is the opinion of Malik. It was narrated from Abu Bakr and Umar (may Allah be pleased with them) that she does not have to observe an 'iddah, and this is the opinion of Al-Thawri, Al-Shafi'i, and the scholars of رائے (personal reasoning/opinion). Therefore, this marriage is valid, and the woman should not pay attention to these whisperings (waswas), and she should beware of giving in to them due to their severe consequences. What helps in getting rid of them is seeking Allah's help, sincerely turning to Him, and supplicating frequently.