What points out to the fact that Shi`as are the followers of the authentic Prophetic Sunnah is hadith al-thaqalayn (tradition of the two weighty things) of the Messenger of Allah who is quoted saying: 

This tradition of the Two Weighty Things in the wording indicated above is recorded by "Ahl al-Sunnah wal Jama`a" in more than twenty of their major Sahih[88] reference books and Musnads[89], and it is included by the Shi`as in all their books of hadith. It is, as you can see, too clear to require any additional clarification in its implication that "Ahl al-Sunnah wal Jama`a" have, indeed, strayed because they did not uphold both of them simultaneously; they have strayed because they preferred their own views to those of Ahl al-Bayt thinking that Abu Hanifah, Malik, al-Shafi`i, and Ibn Hanbal were more knowledgeable than the Pure Progeny, so they followed them and abandoned the Pure Progeny. 

The claim expressed by some of them saying that they uphold the Holy Qur'an is groundless because the Holy Qur'an contains general issues and does not explain its injunctions in detail. It also accepts more than one interpretation. It needs someone to explain and interpret its verses as is the case with the Prophet's Sunnah which requires reliable narrators and knowledgeable interpreters. 

There is no solution for this problem except to refer to Ahl al-Bayt, I mean the Imams from the pure Progeny whom the Messenger of Allah named as his wasis, successors. 

If we add other traditions to the tradition of the Two Weighty Things mentioned above which carry the same meaning and aim at the same goal, such as the following statement of the Prophet: "Ali is with the Qur'an, and the (knowledge of the) Qur'an is with Ali, and they shall never separate till they reach me at the Pool [of Kawthar],"[90] and also his statement, "Ali is with the truth and the truth is with Ali, and they shall never separate from one another till they reach me at the Pool [of al-Kawthar] on the Day of Judgment,"[91] we and any other researcher will then become certain that whoever abandons Ali abandons the true interpretation of the Book of Allah, the Most Exalted One, and whoever forsakes Ali leaves the truth behind his back and follows falsehood, for there is nothing beyond the truth except falsehood. We will also become certain that "Ahl al-Sunnah wal Jama`a" did, indeed, abandon the Holy Qur'an and the Prophet's Sunnah when they abandoned the truth, namely Ali ibn Abu Talib, peace be upon him. This also is a testimony to the accuracy of the Prophet's prophecy indicating that his nation will be divided into seventy-three parties (sects) all of whom will be wrong with the exception of one. 

The saved party is the one that followed the truth and the guidance when it followed Imam Ali. It fought on his side and accepted the peace which only he concluded. It sought guidance from his knowledge then upheld the blessed Imams from his offspring. 

[88] Compilations of traditions they regard as authentic. 

[89] Books upon which religious rulings are based. 

[90] Al-Hakim, Mustadrak, Vol. 3, p. 124, and it is also recorded by al-Dhahabi in his Talkhis. 

[91] Al-Muttaqi al-Hindi, Kanz al-Ummal, Vol. 5, p. 30. Ibn Asakir, Tarikh, Vol. 3, p. 119, Vol. 3.