When I awoke from the dream, I learnt that Rasulullah (sallallahu ‘alaihi wasallam) was declaring nubuwwah and he was secretly inviting to Islam. I thus went to him and met him in the valleys of Makkah Mukarramah. When I reached there, I found that he had just completed performing some Salaah. I then embraced Islam upon his hands. Besides these Sahaabah (Zaid bin Haarithah, Ali bin Abi Taalib and Abu Bakr [radhiyallahu ‘anhum]), no one else had embraced Islam before me. (Usdul Ghaabah vol. 2 pg. 309)

Note:

The darkness in the dream of Hazrat Sa’d (radhiyallahu anhu) referred to the darkness of kufr. The moon with its light referred to Rasulullah (sallallahu alaihi wasallam) who was sent with the light of Islam to remove darkness from the world.

Hazrat Sa’d (radhiyallahu anhu) mentioned that apart from Hazrat Zaid bin Haarithah, Hazrat Ali bin Abi Taalib and Hazrat Abu Bakr (radhiyallahu ‘anhum), no one else had embraced Islam before him. However, this was according to his knowledge. Otherwise, in other narrations, it is established that there were a few other Sahaabah (radhiyallahu ‘anhum) who had embraced Islam before him.”