Sahih Bukhari
Funerals (Al-Janaaiz)
Volume: 002 / Book Number: 023 / Hadith Number: 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.