Sahih al-Bukhari 4123Book 64, Hadith 167
حَدَّثَنَا الْحَجَّاجُ بْنُ مِنْهَالٍ، أَخْبَرَنَا شُعْبَةُ، قَالَ أَخْبَرَنِي عَدِيٌّ، أَنَّهُ سَمِعَ الْبَرَاءَ ـ رضى الله عنه ـ قَالَ قَالَ النَّبِيُّ صلى الله عليه وسلم لِحَسَّانَ " اهْجُهُمْ ـ أَوْ هَاجِهِمْ ـ وَجِبْرِيلُ مَعَكَ " .
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).' "