Ahmad Ibn Hanbal and al-Hakim recorded with authentic document from Abu Said al-Khudri, that the Apostle of God said to Ali: Verily you will do battle for (implementation of) the Quran, as you has done for its revelation. Sunni reference: Tarikh al-Khulafaa, by Jalaluddin al-Suyuti, p173 al-Hakim recorded that Anas Ibn Malik narrated that the Holy Prophet said to Ali:You shall inform my nation about the truth and what they dispute after me: Sunni reference:al-Mustadrak, by al-Hakim, v3, p112, who wrote this is an authentic Hadith according to the stipulation of the two Shaikhs (al-Bukhari and Muslim).[This would mean that the chain of narrators are considered to be authentic as stipulated by Bukhari and Muslim]
The first Muslim It is an undisputed fact that Imam Ali was the first man to embrace Islam after the Holy Prophet (PBUH&HF).Below are some of the references Ibn Abi Shaybah and Ibn Asakir recorded on the authority of Salim Ibn Abi Jaad that he said: I asked of Muhammad Ibn Hanifah, was Abu Bakr the first of the people to adopt Islam? He replied: No. And Ibn Asakir on a reliable ascription from Muhammad Ibn Sa'd Ibn Abi Waqqas, that he said to his father Sa'd: was Abu Bakr al-Siddeeq the first of you in embracing the faith? He said: No, for there were more than five people in faith before him
Ibn Kathir says: it is clear that Muhammad's family believed before every other - his wife Khadija, his freedman Zaid and the Wife of Zaid Umm Ayman and Ali and Warakah. Sunni reference:Tarikh al-Khulafaa, by Jalaluddin al-Suyuti, p33 (History of the Caliphs translated by Major Barrett)
It is also narrated that Anas Ibn Malik said: The Messenger was commissioned on Monday and Ali believed in him on Tuesday. Sunni reference: al-Mustadrak, by al-Hakim, v3, p112 al-Hakim also narrated that:Salman al-Farsi said that the Messenger of God said: The first one of you to drink from the Basin on the Day of Judgment is your first Muslim, Ali, the son of Abu Talib. Sunni reference: al-Mustadrak, by al-Hakim, v3, p112 Ibn Hisham recorded that:Ali Ibn Abi Talib was the first male to believe in the Messenger of God and that he prayed with him while he was only ten years old. Sunni reference: Biography of the Prophet, by Ibn Hisham, v1, p245 The famous Sunni Historian, al-Tabari also wrote:The first three to offer prayers were Muhammad (PBUH&HF) , Khadija and Ali (AS). Sunni reference: History al-Tabari, v2, p65 Also Khateeb al-Baghdadi, in his book quotes Imam Ali that:Ali said: I was the first one to accept Islam at the hands of the Holy Prophet. Tarikh, by al-Khateeb al-Baghdadi, v4, p333