From Sahih Muslim (Hadith 3735):
Safiyya bint Abu 'Ubaid reported on the authority of Hafsa or 'A'isha (Allah be pleased with thein) 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
Reference: Muslim 3735