Sahih al-Bukhari 1300
Sahih al-Bukhari · Book 23: Funerals (Al-Janaa'iz) · Whoever sat down and looked sad when afflicted with a calamity
Narrated Anas:
حَدَّثَنَا عَمْرُو بْنُ عَلِيٍّ ، حَدَّثَنَا مُحَمَّدُ بْنُ فُضَيْلٍ ، حَدَّثَنَا عَاصِمٌ الأَحْوَلُ ، عَنْ أَنَسٍ ـ رضى الله عنه ـ قَالَ قَنَتَ رَسُولُ اللَّهِ صلى الله عليه وسلم شَهْرًا حِينَ قُتِلَ الْقُرَّاءُ، فَمَا رَأَيْتُ رَسُولَ اللَّهِ صلى الله عليه وسلم حَزِنَ حُزْنًا قَطُّ أَشَدَّ مِنْهُ.
When the reciters of Qur'an were martyred, Allah's Messenger (ﷺ) recited Qunut for one month and I never saw him (i.e. Allah's Messenger (ﷺ)) so sad as he was on that day.