Among the first people to embrace Islam

He is among the earliest people to embrace Islam and among the earliest to emigrate (perform Hijrah).  He is among those great people whom the Holy Prophet (Sallallahu alaihi wa sallam) gave the glad tidings of Jannah in this worldly life.  As given in Usdul Ghaba:

Translation: He embraced Islam in the early days of Islam.  Hadhrat Abu Bakr Siddiq (May Allah be well pleased with him) invited him to Islam and he embraced Islam.  He used to say:  Indeed I was the fourth person to embrace Islam.  (Usdul Ghaba Li Ibn Ul Atheer, ‘Uthman Bin ‘Affaan)

It is given in Noor Ul Absaar that he embraced Islam after Hadhrat Abu Bakr Siddiq, Hadhrat Ali and Hadhrat Zaid Bin Haritha (May Allah be well pleased with them).

Translation: ‘Allama Ibn Ishaq (May Allah shower His mercy on him) said:  Hadhrat ‘Uthman is the first one to have embraced Islam after Hadhrat Abu Bakr Siddiq, Hadhrat Ali and Hadhrat Zaid Bin Haritha (May Allah be well pleased with them).  (Noor Ul Absaar Fi Manaqibi Aali Baitin Nabiyyil Mukhtaar Sallallahu alaihi wa sallam, Pg. No. 79)