Prophet Adam was Muslim. All prophets were Muslims. We don't expect rocks, idols or stones with the picture of God to be found because Muslims believe that God isn't like any of his creation.
Many people have the misconception that Islam started about 1,420+ years ago, and that Prophet Muhammad was the founder of the religion of Islam. In fact, Islam existed since man first set foot on this earth.
“Submission” to God existed since man first set foot on Earth. Muhammad isn’t the founder of Islam, but the last and final messenger of Islam. The word “Islam” was first revealed to Muhammad by Allah, but the way of life of ”submission to God’s will” existed since time immorial. Hence, all Prophets of Allah submitted their will to God and abided by the laws which they had to follow. They all preached this fascinating way of life of submission to God.
https://www.youtube.com/watch?v=ByIh7TcrZRA