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:

I was a merchant, and I came during the pilgrimage and stayed with al-'Abbas. While we were with him, a man came out to pray and stood facing the Ka'bah. Then a woman came out and stood praying with him, followed by a youth who stood praying with him. I said, "'Abbas, what is this religion? I do know what this religion is." He answered, "This is Muhammad b. 'Abdallah, who claims that God has sent him as His Messenger with this (religion), and that the treasures of Chusroes and Caesar will be given to him by conquest. This woman is his wife Khadijah bt. Khuwaylid, who has believed in him, and this youth is his cousin 'Ali b. Abi Talib, who has believed in him."

Book  : Tarikh at-Tabari
Author  : Muhammad ibn Jarir al-Tabari (224 AH - 310 AH)
English reference  : Vol. 6, Page 82
Arabic reference  : 
Narrated by: Abu Kurayb, from Yunus b. Bukayr, from Muhammad b. Ishaq, from Yahya b. Abi al-Ash'ath al-Kindi, from a Kufan scholar, from Isma'il b. Iyas b. 'Afif, from his father, from his grandfather.