We assume your questions refers to Rasulullah (Sallallaahu Alayhi Wasallam)
and his Ummah as there were many Muslims in the preceding nations as well.
If so, the Qur’aan states, ‘I (Rasulullah (Sallallaahu Alayhi Wasallam)) am
the first of the believers. The very first person to accept the message of
Rasulullah (Sallallaahu Alayhi Wasallam) was Khadija (Radhiallaahu Anha),
the wife of Rasulullah (Sallallaahu Alayhi Wasallam). The first person to
accept Islam among men was Abu Bakr Siddiqi (Radhiallaahu Anhu).’
and Allah Ta’ala Knows Best
Mufti Ebrahim Desai