"It is not permissible for a woman who believes in Allah and the Last Day to mourn for anyone who dies for more than three days except for a husband; she should mourn for him for four months and ten (days)." (Using translation from Nasāʾī 3503)
عَنْ بَعْضِ أَزَاوِجِ النَّبِيِّ ﷺ قَالَتْ قَالَ رَسُولُ اللهِ ﷺ لَا يَحِلُّ لِامْرَأَةٍ تُؤْمِنُ بِاللهِ وَالْيَوْمِ الْآخِرِ أَوْ تُؤْمِنُ بِاللهِ وَرَسُولِهِ أَنْ تُحِدَّ عَلَى مَيِّتٍ فَوْقَ ثَلَاثٍ إِلَّا عَلَى زَوْجٍ فَإِنَّهَا تُحِدُّ عَلَيْهِ أَرْبَعَةَ أَشْهُرٍ وَعَشْرًا