Shiaism came from Ibn Saba's Cult, not from Quran and Authentic Hadith.
by Kaukab Siddique, PhD
As we look at the Quran and the authentic hadith in Sihah sittah, it is obvious that shia ideas of the superiority of Ali, r.a, over other sahaba is nowhere to be found. The Quran and authentic hadith were accepted by thousands of the sahaba and the two generations of Muslims who followed, known as the tabain and the tubba tabain. Nowhere do we find the divine and superior attributes of Ali propagated by shias. All the thousands of sahaba could not have united to start a conspiracy against Ali. It was physically impossible in that age and time to communicate on such a large scale. Even travel from Makkah took three days and nights.
So why is there such a huge difference between shia and sunni doctrines and worldview. The Islamic narratives, particularly the carefully scrutinized sahih hadith, do project Ali, r.a., as a brave and pious supporter of the Prophet, pbuh, similar to many other Companions of the Prophet, pbuh. No hadith, among the thousands which are known as authentic refer to Ali as superior to all the sahaba .
So from where did the miraculous personality of Ali come into circulation when no agreement or conspiracy among thousands of Sahaba leading to that conclusion can be traced by any scholar.
The most tragic event in the history of Islam, the murder of Usman, r.a., led to upheaval and tumult and war among the Muslims themselves. The "rebels" who murdered Usman, r.a., were well organized and they came from several cities on the margins of the Islamic domain. Tabari, the great Muslim historian wrote about a shadowy figure of Jewish origin who had taken a Muslim name and was known as Abdullah ibn Saba. He was busy moving from city to city and spreading rumors and outright lies against Usman, r.a. The ideas which came to be known in their totality as "shiaism" apparently came from Ibn Saba and his supporters, a cult which came to be known as the Sabaiyyah.
Shias are aware that if Ibn Sabah is recognized as the originator of Shia beliefs, at least in the seminal form, it would destroy Shiaism, coming from a probable Jew who tried to start a religion in total opposition to Islam.
Shias have spent their energies in trying to discredit Ibn Saba and sometimes to even deny his existence.
Shias attack Sayf bin Umar from whom Tabari narrated the report on Ibn Saba. They use the criticism of Sayf by the scholars of hadith who do not accept his credibility. The problem with this argument by Shias is that historical narrations are not judged by the strict standards of hadith. Sayf cannot be accepted in hadith narration but as a journalist he was found acceptable by Tabari who was himself a top notch scholar. Tabari also narrated the story of Karbala and by strict standards of hadith, the Karbala narrative cannot be accepted.
It's essential to note that Tabari was writing in the Abbasid era which was very friendly to Shias and the Ummayad rule was over. So, Tabari was not under anti-Shia pressure from any ruler to accept the narrative of Sayf. 
He is known for his honesty as a historian.
Also, Sayf himself was not narrating his own views but was simply transmitting what was known of the Sabaiyyah myths from his sources and from the pro-Ali cults as such.
What was spread about the cult of Ali, r.a., by Ibn Saba was developed by others after Karbala, such as by Mukhtar who spent his resources and his life in taking revenge for the tragic murder of Hussain, r.a., ibn Ali, r.a.
After Mukhtar came others who developed shia myths seminally touted by Ibn Saba and his followers, the Sabaiyyah.
These myths are now central to Shiism and are totally foreign to the Quran and authentic hadith narrated by sahaba.
Here are some of them:
Ali did not die but was taken up God. Some shias believed that he was God.
The hero not dying will come back to fill the world with justice.
The disappeared hero who will come back is now known to all shias as the Mahdi who will reappear at the end of time.
The myth makers who came after ibn Saba and Mukhtar developed the idea that the Imam in the next generation must be appointed by the Imam In the existing generation. All these imams were supposed to be infallible. [SEE REFERENCES BELOW.]
Abbasid rule began in 750 c.e. Tabari was born in 838 c.e., . and lived for 86 years.
Tabari was not the only scholar who affirmed the existence of Ibn Saba and his influence among the shias. See this independent scholar: Ibn al-Jahiz in Kitab al-Bayan wa al-Tabyin.
Tarikh al-Umam wa al-Muluk by Tabari.
Murawuj al-Dhahab by Masudi. , a shia scholar. The concept of Mahdi does not exist in the Quran. There are 13 "hadith" in Sunni narratives all of which have been debunked as fabricated by Ibn Khaldun in his Muqaddimmah. Of course these do not teach the Mahdi concept as taught by the Shias.
See the collected hadith of Abu Huraira, r.a., in the Musnad of Ahmad ibn Hanbal.