What Happened at Ghadir-e-Khumm?

Tabarani and others have recorded the following tradition as related by Zayd ibn 'Arqam and transmitted through sources unanimously acknowledged to be reliable. Zayd says that the Messenger of Allah, Allah's blessings and peace be upon him and his posterity, delivered a sermon at Ghadir-e-Khumm under a cloth spread as a canopy on two large trees. He said:

  "O my people! I am going to be recalled shortly and I must comply. I shall be interrogated and you also shall be interrogated. What will you say then?" The entire audience answered: "We shall bear witness that you did convey to us the message of Allah, and tried your best to guide us on the right path and always gave us good console. May Allah bless you with a good reward." The Prophet proceeded: "Why do you not bear witness that there is no god but Allah and that Muhammad is His slave and Messenger, and that Paradise is true, Hell is true, Death is true, Resurrection after death is true, that the Day of Judgment will doubtlessly come and that Allah will raise to life the dead from their graves?." They said: "O Yes! We bear witness to all this." Then he said: "O Allah! You also may witness." Then he said: 'O my people! Allah is my Mawla and I am mawla of the faithful and I have superior right on and control over their lives. And this Ali is the mawla of all those of whom I am mawla. O Allah! Love him who loves him and hate him who hates him." He further said: "O my people! I will precede you and you also shall arrive at the pool of Kawthar, the pool wider than the distance between Basrah and San'a', and there are on the pool as many goblets of silver as stars. When you shall reach me I shall interrogate you about your behavior towards the two in-valuable assets after my death. The major asset is the Book of Allah, the Mighty and Glorious, one end of which is in the hand of Allah, the Exalted, and the other end of which in your hands. Grasp it tightly and do not go astray and do not change or amend it. The other asset is my Progeny, who are my Ahl al-Bayt. Allah the Gracious and Omniscient has informed me that the two will not part from each other before they reach me at the pool.