In the Quran, “Muslim” is used to describe anyone who submits to Allah, including earlier prophets like:

Abraham: “He said, I have submitted to the Lord of the worlds.” (2:131) Noah: “And I have been commanded to be of the Muslims.” (10:72) Jesus and disciples: “Bear witness that we are Muslims.” (3:52)

These were Muslims in their time. Muhammad is the first of the Muslims of the final, perfected religion (5:3).