Apollodorus Library 3.14.8

[8]
  Pandion married Zeuxippe, his mother's sister, ¹ and begat two daughters, Procne and Philomela, and twin sons, Erechtheus and Butes. But war having broken out with Labdacus on a question of boundaries, he called in the help of Tereus, son of Ares, from Thrace, and having with his help brought the war to a successful close, he gave Tereus his own daughter Procne in marriage. ² Tereus had by her a son Itys, and having fallen in love with Philomela, he seduced her also saying that Procne was dead, for he concealed her in the country. Afterwards he married Philomela and bedded with her, and cut out her tongue. But by weaving characters in a robe she revealed thereby to Procne her own sorrows. And having sought out her sister, Procne killed her son Itys, boiled him, served him up for supper to the unwitting Tereus, and fled with her sister in haste. When Tereus was aware of what had happened, he snatched up an axe and pursued them. And being overtaken at Daulia in Phocis, they prayed the gods to be turned into birds, and Procne became a nightingale, and Philomela a swallow. And Tereus also was changed into a bird and became a hoopoe.