Hadith: I implore you by Allah (to tell me) if you heard the Messenger of Allah (may Allah's peace and blessings be upon him) saying: Answer on my behalf. O Allah, aid him with the Holy Spirit. He said: By Allah, yes

عن أبي هريرة أنَّ عمر مرَّ بِحَسَّان رضي الله عنهم وهو يَنْشُدُ الشِّعر في المسجد، فَلَحَظَ إليه، فقال: قد كُنْتُ أَنْشُد، وفيه من هو خير مِنْك، ثمَّ الْتَفَتَ إلى أبي هريرة، فقال: أَنْشُدُكَ الله، أَسَمِعْتَ رسول الله صلى الله عليه وسلم يقول: «أَجِبْ عَنِّي، اللَّهُمَّ أَيِّدْهُ بروح الْقُدُسِ»؟ قال: اللهمَّ نعم.
[صحيح] - [رواه مسلم]

Abu Hurayrah reported that ‘Umar happened to pass by Hassān (may Allah be pleased with him) as he was reciting verses of poetry in the mosque. He (‘Umar) looked at him (meaningfully). Upon which, Hassān said: "I used to recite poetry (in the presence) of one better than you (the Prophet). He then looked at Abu Hurayrah and said to him: "I implore you by Allah (to tell me) if you heard the Messenger of Allah (may Allah's peace and blessings be upon him) saying: '(Hassān) answer on my behalf. O Allah, aid him with the Holy Spirit (Jibrīl).’” He (Abu Hurayrah) said: “By Allah, yes (I did).”
[Authentic hadith] - [Narrated by Muslim]

Explanation

The meaning of this Hadīth: Hassān,(may Allah be pleased with him) was reciting poetry in the mosque while ‘Umar was present. ‘Umar looked at him with a disapproving glance. When Hassān noticed, he said to him: "I used to recite poetry in the mosque in the presence of one better than you.” Hassān then asked Abu Hurayrah to testify with his knowledge about Hassān's recitation of poetry in the mosque in the presence of the Messenger (may Allah's peace and blessings be upon him) and his knowledge of his approval and encouragement to it. So ‘Umar said: "I implore you by Allah", meaning: I ask you to swear by Allah, did you hear this from the Messenger of Allah, may Allah's peace and blessings be upon him? When he said: "Answer on behalf of the Messenger of Allah, O Hassān", meaning: answer the (abusing) poetry of the polytheists with your poetry to defend the Prophet (may Allah's peace and blessings be upon him) and aid the religion. Did you hear him say: “O Allah aid him with the Holy Spirit (Jibrīl)", meaning: Jibrīl will aid him by inspiring him with some lines of poetry that strike the nerve of the polytheists like arrows. Abu Hurayrah replied: 'Yes,' meaning: he had heard him reciting poetry in front of the Messenger in the mosque and he heard the Prophet saying that.