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.
Sahih Muslim Book 18, Hadith 84
← Previous Hadith Safiyya bint Abu 'Ubaid narrated this tradition of Allah's Prophet (ﷺ) on the authority of some wives of Allah's Apostle (ﷺ)

Sahih Muslim Book 18, Hadith 83 Next Hadith → A woman must not observe mourning for one who had died for more than three (days) except for four months and ten days in the case of her husband. And she must not wear a dyed garment except one of the types made of dyed yarn, or apply collyrium, or touch perfume except a little perfume or incense, when she has been purified after her courses.

Sahih Muslim Book 18, Hadith 85