(40) باب مَنْ جَلَسَ عِنْدَ الْمُصِيبَةِ يُعْرَفُ فِيهِ الْحُزْنُ
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
In-book reference: Book 23, Hadith 59
USC-MSA web (English) reference(deprecated numbering scheme): Vol. 2, Book 23, Hadith 387