Musannaf Ibn Abi Shayba 34:222

Mu'awiyah ibn Hisham narrated to us, Qays narrated to us, from Salamah ibn Kuhayl, from Abu Sadiq, from 'Ulaym, from Salman, who said: The first of this nation to arrive at its Prophet is the first of them to have accepted Islam: Ali ibn Abi Talib.