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).' "
Sahih al-Bukhari Book 64, Hadith 167
← Previous Hadith Sa`d was wounded on the day of Khandaq (i.e. Trench) when a man from Quraish, called Hibban bin Al-`Araqa hit him (with an arrow). The man was Hibban bin Qais from (the tribe of) Bani Mais bin 'Amir bin Lu'ai who shot an arrow at Sa`d's medial arm vein (or main artery of the arm). The Prophet (ﷺ) pitched a tent (for Sa`d) in the Mosque so that he might be near to the Prophet (ﷺ) to visit. When the Prophet returned from the (battle) of Al-Khandaq (i.e. Trench) and laid down his arms and took a bath Gabriel came to him while he (i.e. Gabriel) was shaking the dust off his head, and said, "You have laid down the arms?" By Allah, I have not laid them down. Go out to them (to attack them)." The Prophet (ﷺ) said, "Where?" Gabriel pointed towards Bani Quraiza. So Allah's Messenger (ﷺ) went to them (i.e. Banu Quraiza) (i.e. besieged them). They then surrendered to the Prophet's judgment but he directed them to Sa`d to give his verdict concerning them. Sa`d said, "I give my judgment that their warriors should be killed, their women and children should be taken as captives, and their properties distributed." Narrated Hisham: My father informed me that `Aisha said, "Sa`d said, "O Allah! You know that there is nothing more beloved to me than to fight in Your Cause against those who disbelieved Your Apostle and turned him out (of Mecca). O Allah! I think you have put to an end the fight between us and them (i.e. Quraish infidels). And if there still remains any fight with the Quraish (infidels), then keep me alive till I fight against them for Your Sake. But if you have brought the war to an end, then let this wound burst and cause my death thereby.' So blood gushed from the wound. There was a tent in the Mosque belonging to Banu Ghifar who were surprised by the blood flowing towards them . They said, 'O people of the tent! What is this thing which is coming to us from your side?' Behold! Blood was flowing profusely out of Sa`d's wound. Sa`d then died because of that."

Sahih al-Bukhari Book 64, Hadith 166 Next Hadith → "On the day of Quraiza's (siege), Allah's Messenger (ﷺ) said to Hassan bin Thabit, 'Abuse them (with your poems), and Jibril is with you.'"

Sahih al-Bukhari Book 64, Hadith 168