Date
The date of this event was the 18th of Dhu"l-Hijjah of the year 10 AH (10 March 632 CE).
Location
After completing his last pilgrimage (Hajjatul-Wada"), Prophet [s] was leaving Makkah toward Madinah, where he and the crowd of people reached a place called Ghadir Khumm (which is close to today"s al-Juhfah). It was a place where people from different provinces used to greet each other before taking different routes for their homes.
The Sermon
Upon receiving the verse, the Prophet [s] stopped on that place (the pond of Khumm) which was extremely hot. Then he sent for all people who have been ahead in the way, to come back and waited until all pilgrims who fell behind, arrived and gathered. He ordered Salman [r] to use rocks and camel toolings to make a pulpit (minbar) so he could make his announcement. It was around noon time in the first of the Fall, and due to the extreme heat in that valley, people were wrapping their robes around their feet and legs, and were sitting around the pulpit, on the hot rocks.
On this day the Messenger of Allah [s] spent approximately five hours in this place; three hours of which he was on the pulpit. He recited nearly one hundred verses from The Glorious Quran, and for seventy three times reminded and warned people of their deeds and future. Then he gave them a long speech.
The following is a part of his speech which has been widely narrated by the Sunni traditionists:
Tradition of the Two Weighty Things (thaqalayn)
The Messenger of Allah [s] declared:
"It seems the time approached when I shall be called away (by Allah) and I shall answer that call. I am leaving for you two precious things and if you adhere to them both, you will never go astray after me. They are the Book of Allah and my Progeny, that is my Ahlul Bayt. The two shall never separate from each other until they come to me by the Pool (of Paradise)."
Acknowledgement of Authority
Then the Messenger of Allah continued:
"Do I not have more right over the believers than what they have over themselves?"
People cried and answered:
"Yes, O" Messenger of God."
Text (nass) of the Designation
Then followed the key sentence denoting the clear designation of "Ali as the leader of the Muslim ummah. The Prophet [s] held up the hand of "Ali and said:
"For whoever I am his Leader (mawla), "Ali is his Leader (mawla)."
http://www.al-islam.org/ghadir/incident.htm