Sahih Muslim | Book 9 | Hadith Number 3549

Narrated by 'Aisha (Allah be pleased with her)
'Aisha (Allah be pleased with her) reported Allah's Messenger (may peace be upon him) as saying: It is not permissible for a woman believing in Allah and the Hereafter to observe mourning on the dead for more than three (days), except in case of her husband.