An eye witness testimony that Ali was the first Muslim after Rasulullah and Khadijah (sa).
Tarikh al-Tabari, Vol. 6, Page 82
Afif's Grandfather:

Al-Abbas b. 'Abd al-Muttalib was a friend of mine. He used to go to the Yemen to buy perfume and sell it during the pilgrimage season. While I was staying with al-'Abbas b. 'Abd al-Muttalib at Mina there came to him a man in the prime of life who performed the ritual ablution thoroughly and then stood praying. Then a woman came out, performed the ablution, and stood by his side praying. I said, "Whatever is this, Abbas" He replied, "This is my brother's son Muhammad b. 'Abdallah b. 'Abd al-Muttalib, who claims that God has sent him as a messenger; this is my brother's son 'Ali b. Abi Talib who has followed him in his religion, and this is Muhammad's wife Khadijah bt. Khuwaylid who has followed him in his religion." After 'Afif had become a Muslim and Islam had firmly rooted in his heart, he used to say, "Would that I had been a fourth."

Book  : Tarikh at-Tabari
Author  : Muhammad ibn Jarir al-Tabari (224 AH - 310 AH)
English reference  : Vol. 6, Page 82
Arabic reference  : 
Narrated by: Ibn Humayd, from Salamah b. al-Fadl and 'Ali b. Mujahid, from Salamah, from Muhammad b. Ishaq, from Yahya b. Abi Ash'ath (Abu Ja'far al-Tabari), from Isma'il b. Iyas b. 'Afif al-Kindi, from his father, from his grandfather.