Sahih al-Bukhari 64:167

Narrated Al-Bara:

The Prophet (ﷺ) 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 Messenger (ﷺ) said to Hassan bin Thabit, 'Abuse them (with your poems), and Gabriel is with you (i.e. supports you).' "