Apollodorus, The Library, 3.1.1 & 3.4..1-2
[3.1.1] Having now run over the family of Inachus and described them from Belus down to the Heraclids, we have next to speak of the house of Agenor. For as I have said, Libya had by Poseidon two sons, Belus and Agenor. Now Belus reigned over the Egyptians and begat the aforesaid sons; but Agenor went to Phoenicia, married Telephassa, and begat a daughter Europa and three sons, Cadmus, Phoenix, and Cilix. But some say that Europa was a daughter not of Agenor but of Phoenix. Zeus loved her, and turning himself into a tame bull, he mounted her on his back and conveyed her through the sea to Crete. There Zeus bedded with her, and she bore Minos, Sarpedon, and Rhadamanthys5; but according to Homer, Sarpedon was a son of Zeus by Laodamia, daughter of Bellerophon.6 On the disappearance of Europa her father Agenor sent out his sons in search of her, telling them not to return until they had found Europa. With them her mother, Telephassa, and Thasus, son of Poseidon, or according to Pherecydes, of Cilix, went forth in search of her. But when, after diligent search, they could not find Europa, they gave up the thought of returning home, and took up their abode in divers places; Phoenix settled in Phoenicia; Cilix settled near Phoenicia, and all the country subject to himself near the river Pyramus he called Cilicia; and Cadmus and Telephassa took up their abode in Thrace and in like manner Thasus founded a city Thasus in an island off Thrace and dwelt there.
[3.4.1] When Telephassa died, Cadmus buried her, and after being hospitably received by the Thracians he came to Delphi to inquire about Europa. The god told him not to trouble about Europa, but to be guided by a cow, and to found a city wherever she should fall down for weariness.31 After receiving such an oracle he journeyed through Phocis; then falling in with a cow among the herds of Pelagon, he followed it behind. And after traversing Boeotia, it sank down where is now the city of Thebes. Wishing to sacrifice the cow to Athena, he sent some of his companions to draw water from the spring of Ares. But a dragon, which some said was the offspring of Ares, guarded the spring and destroyed most of those that were sent. In his indignation Cadmus killed the dragon, and by the advice of Athena sowed its teeth. When they were sown there rose from the ground armed men whom they called Sparti. These slew each other, some in a chance brawl, and some in ignorance. But Pherecydes says that when Cadmus saw armed men growing up out of the ground, he flung stones at them, and they, supposing that they were being pelted by each other, came to blows. However, five of them survived, Echion, Udaeus, Chthonius, Hyperenor, and Pelorus.
[3.4.2] But Cadmus, to atone for the slaughter, served Ares for an eternal year; and the year was then equivalent to eight years of our reckoning.
After his servitude, Athena procured for him the kingdom, and Zeus gave him to wife Harmonia, daughter of Aphrodite and Ares. And all the gods quitted the sky, and feasting in the Cadmea celebrated the marriage with hymns. Cadmus gave her a robe and the necklace wrought by Hephaestus, which some say was given to Cadmus by Hephaestus, but Pherecydes says that it was given by Europa, who had received it from Zeus. And to Cadmus were born daughters, Autonoe, Ino, Semele, Agave, and a son Polydorus. Ino was married to Athamas, Autonoe to Aristaeus, and Agave to Echion