Are there any persons besides the Ummah of Muhammad, sallallaahu 'alayhi wa sallam, who will be saved from Hell?

All perfect praise be to Allaah, the Lord of the worlds. I testify that there is none worthy of worship except Allaah, and that Muhammad, sallallaahu ‘alayhi wa sallam, is His slave and messenger.

After Muhammad was commissioned a Prophet, no person will be saved from Hell except by believing in him. The Prophet said that any Jew or Christian who hears of the Prophet and dies without believing in him will be an inmate of Hell. [Muslim]

As regards the time before he was sent as a Prophet, then whoever believed in the prophet who was sent to them and died while believing in him would enter Paradise. Otherwise, they will be an inmate of Hell.

Allaah Knows best.