The king of Tyre sent his sons Cadmus and his brothers out into the world to find their sister, Europa, but Zeus had carried her off and so they failed in the mission. Instead, the oracle told Cadmus he was to found a (Greek) city (Thebes) where a tired cow lay down. Although Cadmus was new to the area and didn't know about it, the Ismenian dragon guarded a nearby spring (or well) sacred to Ares. Cadmus sent some of his followers to fetch water to make a sacrifice of the exhausted cow, but they didn't return. So Cadmus went to see what was happening for himself. There he saw and slew the threatening dragon responsible for killing Cadmus' men. He used a stone or a spear and then, on the advice of the goddess Athena, planted the teeth of the dragon. These grew into the sown men.
Here's a passage on Cadmus slaying the Isemenian dragon, from the Bibliotheke of Pseudo-Apollodorus:
"[Kadmos] sent some of his men to fetch water from the spring of Ares, but a serpent, said by many to be a child of Ares, guarded the spring and destroyed most of those who had been sent. In outrage Kadmos killed the serpent, and then, following the instructions of Athena, planted its teeth. From this sowing there sprang from the earth armed men, called Spartoi."
Theoi
