مُؤْمِنًا بِهِ
While believing in him.
The encounter must be joined with belief in the Prophet ﷺ. A person who comes intending to embrace Islam and accepts faith in his presence meets this requirement.
A person may also come without that initial intention but accept Islam in the Prophet’s gathering. Once the meeting occurs in a state of faith, the condition is fulfilled. The key issue is belief during the encounter, not merely the person's intention before arriving.