"Shia" means "The Followers"...
"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 (PBUH&HF) 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 , and whatever 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, pp 91-92 Side Comments A Sunni brother mentioned that in the above incedent the Prophet was only addressing his own family Banu Abd al-Muttalib and not the whole of the Muslims.