Sahih al-Bukhari, Hadith 5339 ⓘ
Narrated Um Habiba: The Prophet said, "It is not lawful for a Muslim woman who believes in Allah and the Last Day to mourn for more than three days, except for her husband, for whom she should mourn for four months and ten days."
وَسَمِعْتُ زَيْنَبَ ابْنَةَ أُمِّ سَلَمَةَ، تُحَدِّثُ عَنْ أُمِّ حَبِيبَةَ، أَنَّ النَّبِيَّ صلى الله عليه وسلم قَالَ " لاَ يَحِلُّ لاِمْرَأَةٍ مُسْلِمَةٍ تُؤْمِنُ بِاللَّهِ وَالْيَوْمِ الآخِرِ أَنْ تُحِدَّ فَوْقَ ثَلاَثَةِ أَيَّامٍ، إِلاَّ عَلَى زَوْجِهَا أَرْبَعَةَ أَشْهُرٍ وَعَشْرًا " .
also referenced under: Book 68, Hadith 84 (In-Book) & Volume 7, Book 63, Hadith 252 (USC-MSA)