Sayyidah Umm Atiyyah (may Allah be pleased with her) said: I heard the Messenger of Allah (peace and blessings be upon him) say: "It is not permissible for a woman who believes in Allah and the Last Day to mourn for anyone who has died for more than three days, except for a husband."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 21579