Metamorphoses Book 3: Cadmus
Cadmus was sent out by his father, king of Sidon, to find his sister, Europa. Their father was so distraught over his daughter's kidnapping that he threatened Cadmus with exile if he didn't find her. Cadmus, knowing that he could never trace where Jove had taken the girl, went to the oracle of Apollo and asked where he should make his new home. The god told him that he would meet a cow that had never borne the weight of a yoke or plough, and if Cadmus followed that cow to where she laid down, that's where he should found a new city. Cadmus did as Apollo told him, and everything went according to the prophecy. When Cadmus and his henchmen reached the spot upon which he planned to found a new city, he sent his men into the woods to find water for a sacrifice to Jove.
In the woods, a giant snake guarding the stream killed his men. After they had not come back for a while, Cadmus went to look for them. He found their slain bodies and the great serpent, and after a great struggle, Cadmus killed the snake. He stood staring at the snake when Athene descended and told him not to stare at the snake and warned that one day he would be a snake as well. She told him to take the serpent's teeth and sow them in the ground. He followed her instructions and where he had sown the teeth, full-grown, armed men rose from the ground. Frightened, he raised his weapons, but one of them spoke out against civil strife. With that, the men began to slaughter each other and at the end, all were killed but five who formed a peace pact at Athene's order. These five men became the heads of the five aristocratic families of Thebes, and that's how Thebes was founded.