Bukhari

Hadith Commentary:
An objection has been raised regarding this, that Abu Bakr (radi Allahu anhu), Khadijah (radi Allahu anha), and several others accepted Islam before Sa’d.
Some have said that Sa’d spoke according to his own knowledge, but this is not correct, because Ibn Abd al-Barr (rahimahullah) has narrated from Sa’d that: “I accepted Islam at the age of nineteen, at the hands of Abu Bakr al-Siddiq (radi Allahu anhu). At that time, I was the seventh Muslim.” Some have said that the correct version of this hadith is:
“‘No one accepted Islam on the day that I accepted Islam.’”
Hafiz has said that Ibn Mandah has also narrated this hadith in his work “Ma’rifah” in this very manner. In this case, there remains no difficulty.
(Wahidi)

Hadith Commentary:
The real fact appears to be that in the gathering where Sa’d bin Abi Waqqas radi Allahu anhu accepted Islam, there were only three Muslims present along with the Messenger of Allah sallallahu alayhi wa sallam: one was Abu Bakr radi Allahu anhu, the second was Bilal radi Allahu anhu, and the third was Sa’d bin Abi Waqqas radi Allahu anhu himself. From this, he assumed that there were no other Muslims besides them.

Some have said that on the day he accepted Islam, no one else became Muslim.

Hafiz Ibn Hajar rahimahullah has narrated the wording of the hadith in this manner on the authority of Ibn Mundhir.

In this case, there remains no difficulty, because it is possible that on the day he accepted Islam, no one else became Muslim.
(Fath al-Bari: 7/107)