كتاب الطلاق
18  
The Book of Divorce
(9)
Chapter: 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
(9)
باب وُجُوبِ الإِحْدَادِ فِي عِدَّةِ الْوَفَاةِ وَتَحْرِيمِهِ فِي غَيْرِ ذَلِكَ إِلاَّ ثَلاَثَةَ أَيَّامٍ
Sahih Muslim 1487, 1488 b

This hadith was narrated by Zainab from her mother and from Zainab, the wife of Allah's Apostle (ﷺ), or from some other lady from among the wives of the Prophet (ﷺ).

وَحَدَّثَتْهُ زَيْنَبُ، عَنْ أُمِّهَا، وَعَنْ زَيْنَبَ، زَوْجِ النَّبِيِّ صلى الله عليه وسلم أَوْ عَنِ امْرَأَةٍ مِنْ بَعْضِ أَزْوَاجِ النَّبِيِّ صلى الله عليه وسلم ‏.‏
Reference : Sahih Muslim 1487, 1488b
In-book reference : Book 18, Hadith 75
USC-MSA web (English) reference : Book 9, Hadith 3540
  (deprecated numbering scheme)