Sahih Bukhari Volume 2, Book 23, Hadith 1283

ثُمَّ دَخَلْتُ عَلَى زَيْنَبَ بِنْتِ جَحْشٍ حِينَ تُوُفِّيَ أَخُوهَا، فَدَعَتْ بِطِيبٍ فَمَسَّتْ ثُمَّ قَالَتْ مَا لِي بِالطِّيبِ مِنْ حَاجَةٍ، غَيْرَ أَنِّي سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم عَلَى الْمِنْبَرِ يَقُولُ ‏"‏ لاَ يَحِلُّ لاِمْرَأَةٍ تُؤْمِنُ بِاللَّهِ وَالْيَوْمِ الآخِرِ تُحِدُّ عَلَى مَيِّتٍ فَوْقَ ثَلاَثٍ، إِلاَّ عَلَى زَوْجٍ أَرْبَعَةَ أَشْهُرٍ وَعَشْرًا ‏"

" Later I went to Zainab bint Jahsh when her brother died; she asked for some scent, and after using it she said, "I am not in need of scent but I heard Allah's Apostle saying, 'It is not legal for a woman who believes in Allah and the Last Day to mourn for more than three days for any dead person except her husband, (for whom she should mourn) for four months and ten days.'"