Sahih Muslim 1491
Sahih Muslim · Book 18: The Book of Divorce · The Obligation to mourn during the 'Iddah following the death of one's husband, but it is forbidden to mourn for more than three days in other cases
'A'isha (Allah be pleased with her) reported Allah's Messenger (ﷺ) as saying:
وَحَدَّثَنَا يَحْيَى بْنُ يَحْيَى ، وَأَبُو بَكْرِ بْنُ أَبِي شَيْبَةَ وَعَمْرٌو النَّاقِدُ وَزُهَيْرُ بْنُ حَرْبٍ - وَاللَّفْظُ لِيَحْيَى - قَالَ يَحْيَى أَخْبَرَنَا وَقَالَ الآخَرُونَ، حَدَّثَنَا سُفْيَانُ بْنُ عُيَيْنَةَ ، عَنِ الزُّهْرِيِّ ، عَنْ عُرْوَةَ ، عَنْ عَائِشَةَ ، عَنِ النَّبِيِّ صلى الله عليه وسلم قَالَ " لاَ يَحِلُّ لاِمْرَأَةٍ تُؤْمِنُ بِاللَّهِ وَالْيَوْمِ الآخِرِ أَنْ تُحِدَّ عَلَى مَيِّتٍ فَوْقَ ثَلاَثٍ إِلاَّ عَلَى زَوْجِهَا " .
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.