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.
source : http://www.al-islam.org
