An eye witness testimony that Ali was the first Muslim after Rasulullah and Khadijah (sa).
Tarikh al-Tabari, Vol. 6, Page 83
Narrated Abu Hazim al-Madani and al-Kalbi:

Ali was the first to accept Islam. According to al-Kalbi, he accepted Islam at the age of nine.

Book  : Tarikh at-Tabari
Author  : Muhammad ibn Jarir al-Tabari (224 AH - 310 AH)
English reference  : Vol. 6, Page 83
Arabic reference  : 
Narrated by: Ibn Humayd, from Isa b. Sawadah b. al-Ja'd, from Muhammad b. al-Munkadir, from Rabi'ah b. Abi 'Abd al-Rahman, from Abu Hazim al-Madani and al-Kalbi.