Sahih Al Bukhari Hadith Number 1300

Narrated Anas: When the reciters of Qur’an were martyred, Allah’s Messenger (peace be upon him) recited Qunut for one month and I never saw him (i.e. Allah’s Messenger (peace be upon him) so sad as he was on that day.

23  Funerals (Al-Janaa’iz)
Hadith number 1300
Volume no 2
Chapter: 40 (Whoever sat down and looked sad when afflicted with a calamity)