Someone else in this thread has suggested Lucian's True History as an example of premodern science fiction. I would like to complicate that characterization.

The True History/True Story is a fictional account of traveling to the moon, meeting aliens, and getting involved in an interstellar war. Thus it shares some of its content with modern science fiction. But it does not purport to represent a possible future, or indeed a possible world at all. The genre of ancient literature into which it falls is satire, and its main goal, besides being entertaining, is to parody unreliable/unbelievable history and travel writing. The idea is that going to other celestial bodies is absurd and he's writing about it as though it really happened, just the way someone would write about going to unknown islands and meeting strange peoples there.

Likewise we can look to some other ancient Greek texts and see that, while content associated with science fiction is present, the meaning is completely different. Hephaestus, as a god associated with metalworking and artifice, is described in the Iliad as making robots. He has wheeled tripods that move autonomously to bring things to people, servants that act just like real people but are made of gold, and bellows that can be controlled by spoken commands. Here it is possible that ancient poets or readers had in mind a vision of what human technology could be, but it would have been read primarily as an imagining of what real-world art and technology would look like when combined with divine magic.

There is also a wonderful answer by u/epicyclorama on a thread about premodern time travel narratives that you might be interested in.