Sahih Muslim Book 9, Hadith Number 3545.

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

Safiyya bint Abu ‘Ubaid reported on the authority of Hafsa or ‘Aisha (Allah be pleased with them) or from both of them that Allah’s Messenger (may peace he upon him) said: It is not permissible for a woman believing in Allah and the Hereafter (or believing in Allah and His Messenger) that she should observe mourning for the dead beyond three days except in case of her husband.