Answer: after 10 years of Hegira, messenger of God-my God bless him and his household-decided to perform pilgrimage’s journey and informed everybody. For being with him in Rites of pilgrimage and following him, many groups came to Medina.
This messenger of God’s pilgrimage –may God bless him and his household-was named after that Hegira, these names are: Farewell pilgrimage, Islam pilgrimage, declare pilgrimage, perfect pilgrimage and complete pilgrimage (1).
On Saturday [there remained 5 or 6 nights of Dhu al-Qa’dah or 5 or 6 days to crescent of Dhu al-Hijjat ],Messenger of God wearing only Ihram (white cloth worn by the pilgrims to Mecca ),performed ritual ablution and Tadhin (2) left Medina on foot and also took his wives by camel-litter.
All the people of the Household (Ahlu al bayt of prophet), the Emigrates (Muhajirin), the Helpers (Ansar), many of Arab tribes and other various people groups accompanied him (3).
By transmission of Syphilis or Typhoid fever, many people couldn’t go, nevertheless the accompanied people were so crowded that only God knows its number. It had been said their number were up to 90,000.
During pilgrimage, the number of crowd became more; Mecca’s inhabitants and also a crowd from Yemen who had been Ali-Ibn-Abby Talib‘s–peace be upon him- companions and Abu Musa, joined others (4).
The messenger of God performed ritual ablutions and went to Medina with the same great crowd. There was an area named Juhfa at midway which Medina, Egypt and Iraq’s pathways had been branched from there and also a pool was located there.
When they arrived at this pool on Thursday 18th of Dhu"l-Hijjah, God-high be He exalted-sent down a verse by trustworthy Gabriel: O Messenger (Muhammad)! Proclaim (the Message) which has been sent down to you from Your Lord. And if you do not, then you have not conveyed his Message. Allah will protect you from mankind. Verily, Allah guides not the people who disbelieve.
God ordered, prophet must introduce Ali-Ibn-Abby Talib as a symbol of guidance and raised religion flag and must declare everybody his sainthood and that they should obey him.
Messenger of God –May God bless him and his household-commanded to the people who had gone ahead to come back and who were arriving to stay there. Locating five big Moqilan trees together had been made a good place. The place had been cleaned and swept. Nobody could sit down until the others arrival. It was a hot sunny day. People had put some part of their Aba on their head and for extreme heat of ground they put other part under their feet. They had made a shade for messenger of God –may God bless him and his household-by putting a garment on one of the trees. At noon messenger of God –may God bless him and his household-went toward the place and performed prayer with the people.
After performing prayer, he stood on pulpit of camels’ saddles among people and sermonized loudly and said:
Praise belongs to God, to Thee alone we pray for succour. We believe and trust in Him. We seek refugee to God from the evil of soul and bad deeds. None can guide those whom God leaves in error and none can mislead those whom God guides. Bear witness that there is no God but Allah, Alone without partner, and I bear witness that Muhammad –pbuh- is His slave and messenger.
O people! God who is the Most Subtle (Naghz Tadbir) (10), Courteous and the Aware (Well Acquainted with All things) told me: lifetime of every prophet is equal to half of the lifetime of his previous prophet (11), and I’m going to answer true invitation. I have responsibility and so have you. What’s your answer?
And people said: we testify you declared religion and advised us. In this way you made great effort. God gives you finest reward!
“Don’t you testify there is no God but Allah, Alone without partner, His heaven and hell is true, death is true and surely the Day of Resurrection is coming and God will raise dead up?” messenger of God - may God bless him and his household- said.
“We testify” they answered.
“O God! Bear witness!” he said.
Then he said “O people don’t you hear? “
“We do” people answered
“I’ll enter [heaven] and [Kawthar] pool earlier than you and you will come to me near that pool in the future (12). The pool which its width is the distance of San’a(13) to Bosra(14). And silver goblets are as myriad as sky stars. Careful about how you behave toward two precious things” he said.
Somebody said:”O messenger of God - –may God bless him and his household- what are these two <Thaqals> and two <precious things>?
He said :< major Thaqale > is <the book of Allah>; God-may He be Exalted and Glorified-has got one side of this book and the other side is in your hand, if you keep it firmly you will never be strayed. <minor thaqale> is my<Progeny and Ahlu l’-bayt >. God Who is the Most Subtle, Courteous and Well Acquainted with All things told me these things never will separate until they reach me at pool (kawthar) and I also asked my Lord these two things be like this(never separate from each other).
If you surpass them or leave them you will be perished.
Meanwhile messenger of God–may God bless him and his household-raised Ali-Ibn-Abby Talib -pbuh- by his hand so lofty that the whiteness of their armpits appeared and people recognized him. He said: O people! Who has more priority and right over believers than what they have over themselves?
“God and His prophet know much better” they said.
“God is my leader (Mowla) and I am believers’ leader (Mowla) and I have more right over them than themselves. So those whose leader (Mowla) I am, Ali will also be their leader (Mowla)” said Messenger of God. And he repeated this sentence three times. Ahmad Ibne Hanbal said he had repeated this four times.
Then he said: O Lord! Be friendly with the friends of Ali! Be an enemy to the enemies of Ali! Glory out of affection whoever respects Ali out of affection and adds to his glory and grandeur, humiliate whoever humiliates him! Help the helpers of Ali and don’t protect whoever doesn’t protect him. And always bestow upon him the right. Lo! This word should be declared to absents by presents.
Before scattering of the crowd, this verse was sent down by trustworthy of divine revelation:
“I have perfected Your Religion for you, completed My Favour upon you”.
Here Seal of Prophets–may God bless him and his household- says:
God is greatest upon perfection of religion, completion of blessing and God satisfaction upon my divine mission and sainthood of Ali after me.
Then people began to congratulate commander of faithful -pbuh- and Abu-Bakr and Omar congratulated Imam Ali-pbuh- before other companions. Every one said: Huzza O son of Abu talib! You became the leader (Mowla) of every man and woman-who are believers-and me forever.
Ibne-Abbas said: Sainthood of Ali-pbuh- became obligatory for all the people.
It was a summary of the event of Ghadire Khum which Islamic communities are in agreement at this point. And except than this one, there isn’t any other event of Ghadir Khum in the world. And if it’s said <place of Ghadir>, it would refer to well-know <Khum> pool which is near <Juhfa>.
It was the opinion of all precise scholars and researchers (17).

1- The reason of naming <farewell pilgrimage> is this verse: (Ya ayyoha Rasulo…) [al-Maa’idah:67] and also the reason of naming <perfection pilgrimage> and <complete pilgrimage> is this verse: (Alyaouma akmalto lakom…)[Al-Maa’idah:3]
2-  Greasing the body
 3- Al-Tabaqat, Ibne-sa’ad 225:3 [173/2]; Al-Emta’, Moghrabazi: 510; Ershad-Al sari 429:6 [426/9].
4- Al-seirat Al-halabiat 283:3 [257/3], Al-seirat Ahmad Zeini Dahlan 3:3 [143/2]; Tarikh Al-kholafa, Ibne Juzi: forth chapter; Tazakkarat Khavas Al-ommat: [Page 30]; Dayerat Al-ma’aref, Farid Vajdi 524:3.
5- Clarified by some narrators of Ghadir hadith , included Bara Ibn Azeb.
6- Al-maa’idah: 67.
7- It has come in the original text: <VA nahy an samorat khamsa motagharebat dauhat ‘ezam>; <dauhe>: means big tree. Where there is <dauh> and <dauhat>, it refers to this place and at last to the event of Ghadir.
8- Majma Al-zava’ed, Hafez Heithamy 106:9.
9- Thamar Al-gholub: 51[p 636, no 1068].
10- [<subtle>: one of the meanings of subtle is <the Subtle and the Aware> which it seems is appropriate for this dignity and we have translated this in <Naghz Tadbir>; R.K: Majma’o Al-bayan exegesis 128:4, following Al-An’am:103.
11- We didn’t find this phrase in Shii’ah books and also its meaning isn’t clear.
12- [<Thaqaleyn>: clarified by some lexicographers like, Azhari, Tha’lab, Ibne Athir and Firuz-<abadi, Thaqalayn> is correct. Thaqal means <precious and important matter> and also <provision> and considering that its infinitive is <Thaqal>, there are 3 reasons for naming <Quran and progeny> in <Thaqal>:
a) These are known precious and important matters, because of magnifying their importance and dignity.
b) Because performing and holding fast to them makes provision of the day of resurrection.
c) Because performing and holding fast to them are hefty and weighty.
R.K: Tahzib Al-loghat, Azhari; Al-nahayat Fy Gharib Al-hadith, Ibne Athir; Lesan Al-arab, Ibne Manzur; Ghamus Al-mohit, Firuz Abadi: Maddeh (TH Q L)
13- <San’a>: capital of Yemen.
14- It is a village which is suburb of Damascus and is well-known by the name of<Kurat Huran>.
15- <Sainthood> is a kind of friendship which is made for helping and assisting and its opposite word is <animosity>; it means enmity for avoiding from helping and assisting.
16- Al-maa’idah: 3.
17- Comprehensive s e l e c tion of Al-ghadir, page 41.

Source: makarem