Hazrat ‘Uthmaan (radhiyallahu ‘anhu) then said, “O Rasul of Allah (sallallahu ‘alaihi wasallam)! I arrived recently from Shaam. (While we were travelling from Shaam,) when we were between Mu‘aan and Zarqaa’, we were as if asleep when we heard a person announce, ‘O people who are asleep! Wake up, for Ahmed (sallallahu ‘alaihi wasallam) has appeared in Makkah Mukarramah!” When we thereafter arrived in Makkah Mukarramah, we heard of you (claiming to be the Nabi of Allah Ta‘ala).”
Hazrat ‘Uthmaan (radhiyallahu ‘anhu) had accepted Islam in the very early days, before Rasulullah (sallallahu ‘alaihi wasallam) had entered Daar-ul-Arqam.
(Ibn Sa’d 3/40)