Dear inuit,
There were no Sunnis and Shias at the time of Messenger of Allah (upon him be peace and blessings) - They all followed the Messenger of Allah (Upon him be peace and blessings) in detail [His Sunnah].
The Companions of Allah’s Messenger were his students and he taught them the purest form of Islam and they propagated this to their students and so on.
Reported on the authority of `Umran Ibn Husain (ra), that he said: "Allah's Messenger (Upon him be peace and blessings) said:
*"The best of my Ummah is my generation, then those who follow them, then those who follow them." `Umran (ra) said: "I don't know if he mentioned two generations after his, or three." "Then after you," continued the Prophet , "there will come a generation who will testify without being called upon to do so and they will be treacherous and untrustworthy. They will swear oaths, but they will not fulfill them and obesity will be seen in them." * - (Narrated by Muslim)
The Messenger of Allah (Upon him be peace and blessings) informs us in this Hadith that the best of this community of Muslims are his Companions (May Allah be pleased with them all) and the two or three generations who succeeded them due to their adherence to Islam and their correct beliefs and practices and their freedom from hypocrisy, deviation and innovation. History is proof that later generations ‘imported’ innovations into their practice of Islam.
Imam Muslim relates in his Sahih that God’s Messenger (upon him be peace and blessings) declared: *“The stars are means of security for the heaven, [that is, the heaven is maintained because of the delicate order among the stars]. When the stars are scattered [when that order collapses], what was promised for the heaven befalls it *. I am the means of security for my Companions [my Companions will continue to live in peace and security as long as I am alive]. When I leave the world, what was promised for my Companions will befall them. My Companions are means of security for my nation (Ummah). When my Companions leave the world, what was promised for my Ummah will befall them” [they will be exposed to many misfortunes and calamities]. * - – Sahih Muslim, “Fada’il al-Sahaba,” 207
To follow uncorrupted form of Islam we have to follow Islam as taught by Allah's Messenger (Upon him be peace and blessings) to his Companions and the way they understood and practiced it.
Edit: Just to confirm that: They all followed the Messenger of Allah (Upon him be peace and blessings) in detail [His Sunnah].
Below Imam Ali (ra), himself is confirming that there was not an iota of difference in faith between him and his 'opponents'.
---The thing began in this way: We and the Syrians were facing each other while *we had common faith in one Allah, in the same Prophet (s) and on the same principles and canons of religion. So far as faith in Allah and the Holy Prophet (s) was concerned we never wanted them (the Syrians) to believe in anything over and above or other than what they were believing in and they did not want us to change our faith. Both of us were united on these principles. * The point of contention between us was the question of the murder of Uthman. It had created the split. They wanted to lay the murder at my door while I am actually innocent of it................." Nahjul Balagha Letter 58*