Sahih Bukhari Volume 5, Book 59, Hadith Number 449.

Narrated By Al-Bara : The Prophet said to Hassan, “Abuse them (with your poems), and Gabriel is with you (i.e., supports you).” (Through another group of sub narrators) Al-Bara bin Azib said, “On the day of Quraiza’s (besiege), Allah’s Apostle said to Hassan bin Thabit, ‘Abuse them (with your poems), and Gabriel is with you (i.e. supports you).'”