It is narrated from Sayyidah Umm Salamah and Sayyidah Aishah (may Allah be pleased with them both) that they heard the Messenger of Allah (peace and blessings be upon him) say: "It is not lawful for a woman who believes in Allah and the Last Day to mourn for a deceased person for more than three days, except for her husband."
Hadith Referenceالمعجم الكبير للطبراني / مسند النساء / 20145