Lady Aisha Siddiqa (may Allah be pleased with her) and Lady Hafsa (may Allah be pleased with her) narrate that the Prophet Muhammad (peace and blessings be upon him) said: "It is not permissible for any woman who believes in Allah and the Last Day to mourn for anyone who dies for more than three days, except in the case of her husband; for him, the mourning period is four months and ten days."
Hadith Referenceصحیح ابن حبان / كتاب الطلاق / 4302
Hadith Gradingفضيلة الشيخ الإمام محمد ناصر الدين الألبانيصحيح - انظر ما قبله.فضيلة الشيخ العلّامة شُعيب الأرناؤوطإسناده صحيح على شرط مسلم