In his Firaq al-Shi''ah (The Shi''ah Groups), Abu Muhammad al-Hasan bin Musa al-Nubakhti, one of the foremost known Shi''i historians, believes that Shi''ism did not start until the demise of the Holy Rasul, Sallallahu alayhi wa Sallam:
"The Messenger [sawa] died in the month of Rabi'' al-Awwal, in the year 10 of Hijra at age 63 and the duration of his prophethood was for 23 years, and his mother is Aaminah bint Wahab bin Abdi Manaaf bin Zuhra bin Kilaab bin Murra bin Ka''b bin Lu''ay bin Ghaalib. (At his death) The Ummah was divided into three groups. One group was called the Shi''ites, who were the Shi''ites of Ali bin Abi Taleb [as] and from them all Shi''i sects broke away. Another (group) claimed the right of succession, i.e., al-Ansaar, who called for the inauguration of Sa''d bin Ubadah al-Khazraji. A (Third) group tilted toward giving the Bay''ah (allegiance) to Abu Bakr bin Abi Qahaafah, with an excuse that the Messenger [sawa] did not name a particular successor rather left it for the Ummah to chose whom it wills...." [Firaq ash-Shi''ah: pp. 23-24]
So al-Nubakhti is stating:
Shia''ism was formed upon the death of Rasulullah (s)
Shi''as were those individuals who attached themselves to Hadhrath ''Ali (as)
Remember Rasulullah (s) had already said that ''Ali and his Shi''a would attain Paradise. Clearly during the life of Rasulullah (s) there was no reason for a separate group to call itself the Shi''a of ''Ali. The Shi''as were those who right from the beginning viewed Imam Ali (as) the rightful successor of Rasulullah (s) and attached themselves to him. Hence that group that deemed Imam Ali (as) to be rightful successor of Rasulullah (s) and affiliated themselves with his cause were indeed his supporters - his Shi''a.
