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 1490 b

A hadith like this is transmitted on the authority of Nafi'.

وَحَدَّثَنَاهُ شَيْبَانُ بْنُ فَرُّوخَ، حَدَّثَنَا عَبْدُ الْعَزِيزِ، - يَعْنِي ابْنَ مُسْلِمٍ - حَدَّثَنَا عَبْدُ، اللَّهِ بْنُ دِينَارٍ عَنْ نَافِعٍ، بِإِسْنَادِ حَدِيثِ اللَّيْثِ ‏.‏ مِثْلَ رِوَايَتِهِ ‏.‏
Reference : Sahih Muslim 1490b
In-book reference : Book 18, Hadith 81
USC-MSA web (English) reference : Book 9, Hadith 3546
  (deprecated numbering scheme)