Rasulullah (sallallahu ‘alaihi wasallam) replied by reciting the following khutbah:

الحمد لله نحمده ونستعينه ونستغفره ونعوذ بالله من شرور أنفسنا من يهده الله فلا مضل له ومن يضلله فلا هادي له وإني أشهد أن لا إله إلا الله وحده لا شريك له وأشهد أن محمدا عبده ورسوله

All praise is due to Allah! We praise Him, ask Him for assistance and beg His forgiveness. We seek the refuge of Allah from the evils of our base desires. He whom Allah guides, none can lead him astray, and he whom Allah leads astray, none can guide him. I bear witness that there is none worthy of worship but Allah who is alone and has no partner, and I bear witness that Muhammed (sallallahu ‘alaihi wasallam) is His slave and messenger.

Hazrat Dimaad (radhiyallahu ‘anhu) relates:

I requested Rasulullah (sallallahu ‘alaihi wasallam) to repeat his words again. By Allah! I had heard abundant poetry and many incantations of fortune-tellers, but by Allah, never before had I heard such words. These words are submerged in the depths of the ocean of eloquence. I immediately embrace Islam and I also declare that I bear testimony that there is none worthy of worship but Allah who is alone and has no partner and I bear witness that Nabi Muhammed (sallallahu ‘alaihi wasallam) is His slave and messenger.

In this manner, Hazrat Dimaad (radhiyallahu ‘anhu) embraced Islam, and he also pledged allegiance at the hands of Rasulullah (sallallahu ‘alaihi wasallam) on behalf of his people.

(Extracted from Seeratul Mustafa 1/208-209)