Safiyyah bint Abi Ubayd narrates that Hafsah (may Allah be pleased with her) said that the Messenger of Allah (peace and blessings be upon him) said: "It is not permissible for a woman who believes in Allah and the Last Day, or believes in Allah and His Messenger, to mourn for a deceased person for more than three days, except for her husband."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19664