Teaching in Madinah
Muhammad ﷺ sent him to teach the new believers in Madinah before the Hijrah. With As‘ad ibn Zurara’s support, he taught the Qur’an and reached influential members of the Aws and Khazraj.
Source: TDV Encyclopaedia of Islam — Mus‘ab ibn Umayr
Uhud
After the migration, Mus‘ab continued serving the community. He carried the banner at Uhud and was killed in 625. Accounts of his burial contrast his final material circumstances with the wealth of his youth.
Source: TDV Encyclopaedia of Islam — Mus‘ab ibn Umayr