Sahih Muslim Book 9, Hadith Number 3549.

Chapter: It is obligatory to abstain from adornment during the ‘Idda period, but mourning beyond three days is prohibited.

‘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.