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) , 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."

Sunni reference: Tarikh, by al-Khateeb al-Baghdadi, v4, p333