Sahih al-Bukhari 1300

Arabic

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.‏

English translation

Narrated Anas: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

Reference: Sahih al-Bukhari 1300, Book of Funerals (Al-Janaa'iz). English translation by Dr. Muhammad Muhsin Khan.