Sahih Muslim
The Book of Divorce (Kitab Al-Talaq)
Book Number: 009 / Hadith Number: 3549
'A'isha (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.