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.
Sahih Muslim Book 18, Hadith 80
← Previous Hadith I had in fact no need of it, but I heard Allah's Messenger (ﷺ) as saying: It is not permissible for the women believing in Allah and the Hereafter to abstain from adornment beyond three days except (at the death of) husband (in which case she must abstain from adornment) for four months and ten days.

Sahih Muslim Book 18, Hadith 79 Next Hadith → A hadith like this is transmitted on the authority of Nafi'.

Sahih Muslim Book 18, Hadith 81