Imamat of Ali (A.S) Announced During the Inception of Islamic Mission

The following two traditions are written, one after the other, in the History of al-Tabari which is one of the important history book for the Sunnis. Besides al-Tabari, many other historians and traditionists and commentators of Quran from among Sunnis have recorded this tradition in their books. (see below for the list of references). The two traditions explicitly indicate that the Holy Prophet of Islam (peace be upon him and his family), with the order of Allah, has introduced Ali Ibn Abi Talib as his successor even in his first open preach to the people:

Narrated Ibn Humayed, from Salamah, from Muhammad Ibn Is'haq, from Abd al-Ghaffar Ibn al-Qasim, from al-Minhal Ibn Amr, from abdallah Ibn al-Harith Ibn Nawfal Ibn al-Harith Ibn Abd al-Muttalib, from Abdallah Ibn Abbas, from Ali Ibn Abi Talib:

When the verse "And warn your close tribe (Quran 26:214)" was revealed to the prophet, he called me and said to me, "Ali, God has commanded me to warn my tribe of near kindred. I was troubled by this, since I knew that when I discuss the matter to them they would respond in a way which I would not like. I kept silent until Gabriel came to me and said "If you do not do what you are commanded, your Lord will punish you." So prepare a measure of wheat for us, add a leg of lamb to it, fill a large bowl of milk for us, and then invite sons of Abd al- Muttalib for me so that I may speak to them what I have been commanded to tell them."

I did what he had told me to do. At that time they numbered forty men more or less, including his uncles Abu Talib, Hamzah, al-Abbas, and Abu Lahab. When they had gathered together, he called me to bring the food which I had prepared. I brought it, and when I put it down, prophet took a piece of meat, broke it with his teeth, put it in the dish. Then he said, "Take in the name of God." They ate until they could eat no more, and yet the food was as it had been.

I swear by God, in whose hand Ali's soul rests, that a single man could have eaten the amount of food which I prepared for them. Then he said "Give them something to drink." So I brought them the bowl and they drank from it until they became full, and I swear by God that one man could have drunk that amount. When prophet wanted to speak to them, Abu Lahab interrupted him and said "Your host has long since bewitched you." Then they dispersed without the prophet speaking to them.

On the following day he said to me "Ali, this man interrupted what I wanted to say so that people dispersed before I could speak to them. Prepare the same food for us as you did yesterday, and invite them here." I did this, and brought them food when he called me. He did as he had done the other day, and they ate until they could eat no more. Then he said, "Bring the bowl," and they drank until they could drink no more.

Then he spoke to them, saying, "Banu Abd al-Muttalib, I don't know of any young man among Arabs who has brought for his people something better than what I have brought to you. I bring the best of this world and the world after, since God has commanded me to summon you to him. Which of you will aid me in this matter, so that he will be my brother, my executor (Wasi), my successor (Caliph) among you?" They all held back, and even though I was the youngest, I said "I will be your helper, O' prophet of God."

He put his hand on the back of my neck and said "This is my brother, my executor (Wasi), my successor (Caliph) among you, so listen to him and obey him." They rose up laughing and saying to Abu Talib, "He has commanded you to obey your son and to obey him!

Sunni References:

(1) History of al-Tabari, English version, v6, pp 88-91
(2) History of Ibn Athir, v2, p62
(3) History of Ibn Asakir, v1, p85
(4) Tafsir al-Durr al-Manthoor, by al-Hafidh Jalaluddin al-Suyuti, v5, p97
(5) Tafsir al-Khazin, by Ala'uddin al-Shafi'i, v3, p371
(6) Shawahid al-Tanzil, by al-Hasakani, v1, p371
(7) Kanz al-Ummal, by al-Muttaqi al-Hindi, v15, p15, pp 100-117
(8) al-Sirah al-Halabiyah, v1, p311
(9) Dala'il al-Nabawiyyah, by al-Baihaqi, v1, pp 428-430
(10) al-Mukhtasar, by Abul Fida, v1, pp 116-117
(11) Life of Muhammad, by Hasan Haykal, p104 (First Arabic Edition only. In
the second edition the last sentence of Prophet is deleted.)
(12) Tahdhib al-Athar, v4, pp 62-63.
The above tradition was also narrated by important Sunni figures such as Muhammad Ibn Is'haq, Ibn Abu Hatem, and Ibn Mardawayh. It is also recorded by many orientalists including T. Carlyle, E. Gibbon, J. Davenport, and W.
Irving.
As we see, Prophet ordered people to LISTEN AND FOLLOW ALI even in his first open preach, that is, when he declared his prophethood openly. "Shia" means "The Followers", and it is exclusively used for "The Followers of Imam Ali".

Thus Shia school of thought was in fact established by the Holy Prophet of Islam (S.A.W.) from the beginning of his mission.
If we follow Imam Ali (AS), this is because the Holy Prophet asked us to do so. Moreover, whatever Imam Ali (AS) (and other 11 Imams) has said is the exact teachings and sayings of the Prophet Muhammad, and whatever Prophet Muhammad has said is the exact teachings and sayings of Allah.

This is because the prophets and Imams are infallible and they do not say anything in contrary to what they have been commanded to say.

The next tradition in the History of al-Tabari is as follows:

Narrated Zakariyya Ibn Yahya al-Darir, from Affan Ibn Muslim, from Abu Awanah, from Uthman Ibn al-Mughirah, from Abu Sadiq, from Rabiah Ibn Najid:

A man said to Ali: "O Commander of believers, how did you become the heir of your cousin to the exclusion of your paternal uncle?" Ali said:"Ahem" three times until every body craned their necks and pricked up their ears, and then said "Prophet invited the whole of the Banu Abd al-Muttalib, including his own closest relatives, to eat a year old lamb and to drink some milk. He also prepared a quantity of wheat for them, and they ate until they were full, while the food remained as it was, as though it had not been touched. Then he called for a drinking cup and they drank until they could drink no more, while the drink remained as though it had not been touched and they had not drunk. Then he said Banu abd al Muttalib, I have been sent to all men in general and to you in particular. Now that you have seen what you have seen, which of you will swear an oath of allegiance to me to become my brother, my companion, and my inheritor? Not one of them rose up, so I stood up before him even though I was the youngest there. He said Sit down. He repeated the words he had spoken for three times while I would rise up and he would say to me sit down. On the third occasion, he struck his hand on mine. In this way I became the heir of my cousin to the exclusion of my uncle.
Sunni References: History of al-Tabari, English version, v6,

Sunni reference: Tarikh, by al-Khateeb al-Baghdadi