Language:
كتاب الطلاق
68  
Divorce
(46)
Chapter: A widow should mourn for four months and ten days.
(46)
باب تُحِدُّ الْمُتَوَفَّى عَنْهَا زَوْجُهَا أَرْبَعَةَ أَشْهُرٍ وَعَشْرًا
Sahih al-Bukhari 5336

Zainab further said:

"I heard my mother, Um Salama saying that a woman came to Allah's Messenger (ﷺ) and said, "O Allah's Messenger (ﷺ)! The husband of my daughter has died and she is suffering from an eye disease, can she apply kohl to her eye?" Allah's Messenger (ﷺ) replied, "No," twice or thrice. (Every time she repeated her question) he said, "No." Then Allah's Messenger (ﷺ) added, "It is just a matter of four months and ten days. In the Pre-Islamic Period of ignorance a widow among you should throw a globe of dung when one year has elapsed."

قَالَتْ زَيْنَبُ وَسَمِعْتُ أُمَّ سَلَمَةَ، تَقُولُ جَاءَتِ امْرَأَةٌ إِلَى رَسُولِ اللَّهِ صلى الله عليه وسلم فَقَالَتْ يَا رَسُولَ اللَّهِ إِنَّ ابْنَتِي تُوُفِّيَ عَنْهَا زَوْجُهَا وَقَدِ اشْتَكَتْ عَيْنَهَا أَفَتَكْحُلُهَا فَقَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ لاَ ‏"‏‏.‏ مَرَّتَيْنِ أَوْ ثَلاَثًا كُلَّ ذَلِكَ يَقُولُ لاَ، ثُمَّ قَالَ رَسُولُ اللَّهِ صلى الله عليه وسلم ‏"‏ إِنَّمَا هِيَ أَرْبَعَةُ أَشْهُرٍ وَعَشْرٌ، وَقَدْ كَانَتْ إِحْدَاكُنَّ فِي الْجَاهِلِيَّةِ تَرْمِي بِالْبَعَرَةِ عَلَى رَأْسِ الْحَوْلِ ‏"‏‏.‏
Reference : Sahih al-Bukhari 5336
In-book reference : Book 68, Hadith 81
USC-MSA web (English) reference : Vol. 7, Book 63, Hadith 251
  (deprecated numbering scheme)