Volume 2 · Book 23 · Hadith 387
Narrated Anas: When the reciters of Quran were martyred, Allah's Apostle recited Qunut for one month and I never saw him (i.e. Allah's Apostle) so sad as he was on that day.
— Sahih Bukhari, Volume 2, Book 23: Funerals (Al-Janaa'iz), Hadith #387