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

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.