According to the narration of Mustadrak Haakim, when Hazrat Hamzah (radhiyallahu ‘anhu) appeared before Rasulullah (sallallahu ‘alaihi wasallam), he declared:
أشهد أنك لصادق شهادة المصدق والعارف
I give the testimony of one who has belief and knows what he is testifying to that you are definitely the true Nabi.
He then said, “O my nephew! You may proclaim your deen in public. By Allah! Even if I am offered the whole world and whatever is contained in it, I will not renounce your religion and prefer the religion of my forefathers.”
Saying this, he composed the following poetry:
حمدت الله حين هدى فؤادي إلى الإسلام والدين الحنيف
I praise Allah who has guided my heart towards Islam and towards the straight religion (which conforms to the way of Nabi Ebrahim (‘alaihis salaam))
لدين جاء من رب عزيز خبير بالعباد بهم لطيف
He guided me towards that deen which came from the powerful Lord who has complete knowledge of His servants and is extremely kind to them.
إذا تليت رسائله علينا تحدر دمع ذي اللب الحصيف
When His messages are recited before us, then the tears of those who are complete in intelligence begin to flow (as they understand the message)
رسائل جاء أحمد من هداها بآيات مبينة الحروف
(They are) messages brought by Ahmed (sallallahu ‘alaihi wasallam) for the guidance of the people, messages that are completely clear.
وأحمد مصطفى فينا مطاع فلا تغشوه بالقول العنيف
And Ahmed (sallallahu ‘alaihi wasallam) is the chosen one amongst us who should be obeyed, so do not conceal it (the message) with harsh words.
فلا والله نسلمه لقوم ولما نقض فيهم بالسيوف
By Allah! We will never hand him over to anyone, so long as swords do not make the final judgement between us (until we lose our lives defending him).
Through Hazrat Hamzah (radhiyallahu ‘anhu) embracing Islam, the Quraish understood that it would not be easy for them to go ahead with their plans and try to cause harm to Rasulullah (sallallahu ‘alaihi wasallam).
(Extracted from Seeratul Mustafa 1/184-185)