It is narrated from Sayyidah Hafsah (may Allah be pleased with her) 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 to mourn for a deceased person for more than three days, except for a husband."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 19639