Sahih al-Bukhari 5335
Sahih al-Bukhari · Book 68: Divorce · A widow should mourn for four months and ten days.
Zainab further said:
قَالَتْ زَيْنَبُ فَدَخَلْتُ عَلَى زَيْنَبَ ابْنَةِ جَحْشٍ حِينَ تُوُفِّيَ أَخُوهَا، فَدَعَتْ بِطِيبٍ فَمَسَّتْ مِنْهُ، ثُمَّ قَالَتْ أَمَا وَاللَّهِ مَا لِي بِالطِّيبِ مِنْ حَاجَةٍ غَيْرَ أَنِّي سَمِعْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم يَقُولُ عَلَى الْمِنْبَرِ " لاَ يَحِلُّ لاِمْرَأَةٍ تُؤْمِنُ بِاللَّهِ وَالْيَوْمِ الآخِرِ أَنْ تُحِدَّ عَلَى مَيِّتٍ فَوْقَ ثَلاَثِ لَيَالٍ إِلاَّ عَلَى زَوْجٍ أَرْبَعَةَ أَشْهُرٍ وَعَشْرًا " .
I want to Zainab bint Jahsh when her brother died. She asked for perfume and used some of it and said, "By Allah, I am not in need of perfume, but I have heard Allah's Messenger (ﷺ) saying on the pulpit, 'It is not lawful for a lady 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.'"