Sahih al-Bukhari, Book 23, Hadith 59 (In-Book) ⓘ
Narrated Anas: When the reciters of Qur'an were martyred, Allah's Messenger recited Qunut for one month and I never saw him (i.e. Allah's Messenger) so sad as he was on that day.
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.
also referenced under: Hadith 1300 (DarusSalam) & Volume 2, Book 23, Hadith 387 (USC-MSA)