"It is not permissible for a woman who believes in Allah and the Last Day to mourn for more than three days, except for her husband." (Using translation from Nasāʾī 3526)
أَنَّ رَسُولَ اللهِ ﷺ قَالَ «لَا يَحِلُّ لِامْرَأَةٍ تُؤْمِنُ بِاللهِ أَنْ تُحِدَّ فَوْقَ ثَلَاثَةِ أَيَّامٍ إِلَّا عَلَى زَوْجٍ»