Surah 18:86
"until, when he reached the setting of the sun, he found it setting in a muddy spring, and he found next to it a people. We said, “Dhū-l-Qarnayn! Either punish (them) or do them (some) good.”"
Cosmology / Flat Earth
The verse describes an explorer traveling to the literal, physical edge of the earth where he finds the physical location where the sun sets into a muddy spring. This necessitates a flat earth model, as on a spherical rotating Earth, the sun does not physically set into a terrestrial location.