It is narrated from Sayyidah Aishah (may Allah be pleased with her) that he (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 her husband."
Hadith Referenceالسنن الكبرى للبيهقي / كتاب العدد / 15522