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.

A hadith like this is transmitted on the authority of Nafi'.

" She should abstain from adorning herself (in case of the death of her husband) for four months and ten days."

Safiyya bint Abu 'Ubaid narrated this tradition of Allah's Prophet (ﷺ) on the authority of some wives of Allah's Apostle (ﷺ)

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.

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.

A hadith like this has been narrated on the authority of Hisham with the same chain of narrators but with a slight variation of words.

We were forbidden to observe mourning for the dead beyond three days except in the case of husband (where it is permissible) for four months and ten days, and (that during this period) we should neither use collyrium nor touch perfume, nor wear dyed clothes, but concession was given to a woman when one of us was purified of our courses to make use of a little incense or scent.