slam believes in the unseen. We don't believe in ghosts.
Let me specify. What is a "ghost?" Typically, this means "the soul or spirit of a dead person who haunts the world of the living." This definition contradicts Islam; once a soul leaves, it never returns until the Day of Resurrection.
There are many ayaat and ahadith that talk about what happens to a human soul when it dies. To summarize:
The angel of death comes and extracts the soul
The angels wrap it and carry it
It is eventually returned to its grave
The person is asked the questions of the grave
They remain there until resurrection day
Going back to ghosts: on the other hand, we have Jinns, which are mentioned in the Qur'an: invisible beings of great physical strength. They can possess (enter into and speak on behalf of) humans.
Among the proofs of this are: