Immediately after the Prophet's speech, Hassan b. Thabit, the Companion and poet of the Messenger of Allah [s], asked for his permission to compose a few verses of poetry about Imam 'Ali [a] for the audience. The Prophet [s] said: "Say with the blessings of Allah". Hassan stood up and said: "O' people of Quraysh. I follow with my words what preceded and witnessed by the Messenger of Allah [s]. He then composed the following verses at the scene:
He calls them, (on) the day of Ghadir, their Prophet
In Khumm so hear (and heed) the Messenger's call,
He said: "Who is your guide and leader? (mawlakum wa waliyyukum)"
They said, and there was no apparent blindness (clearly):
"Your God, our guide, and you are our leader
And you won't find from among us, in this, any disobedient,"
He said to him: "Stand up O' Ali, for I am
pleased to announce you Imam and guide after me (min ba'di imam(an) wa hadiy(an)),
So whomever I was his leader (mawla), then this is his leader (mawla)
So be to him supporters in truth and followers,"
There he prayed: "Allah! Be a friend and guide to his follower
And be, to the one who is Ali's enemy, an enemy"
source : http://www.al-islam.org
