Homeric Hymn to Demeter 154:
…and Polykseinos and blameless Eumolpos… (translated by Aaron J. Ivey)
Homeric Hymn to Demeter 475:
and to doughty Eumolpus and Celeus, leader of the people, she showed the conduct of her rites and taught them all her mysteries (original Greek).
Apollodorus, Library 3.15.4:
Chione had connexion with Poseidon, and having given birth to Eumolpus unknown to her father, in order not to be detected, she flung the child into the deep. But Poseidon picked him up and conveyed him to Ethiopia, and gave him to Benthesicyme（a daughter of his own by Amphitrite） to bring up. When he was full grown, Benthesicyme’s husband gave him one of his two daughters. But he tried to force his wife’s sister, and being banished on that account, he went with his son Ismarus to Tegyrius, king of Thrace, who gave his daughter in marriage to Eumolpus’s son. But being afterwards detected in a plot against Tegyrius, he fled to the Eleusinians and made friends with them. Later, on the death of Ismarus, he was sent for by Tegyrius and went, composed his old feud with him, and succeeded to the kingdom. And war having broken out between the Athenians and the Eleusinians, he was called in by the Eleusinians and fought on their side with a large force of Thracians. When Erechtheus inquired of the oracle how the Athenians might be victorious, the god answered that they would win the war if he would slaughter one of his daughters; and when he slaughtered his youngest, the others also slaughtered themselves; for, as some said, they had taken an oath among themselves to perish together. In the battle which took place after the slaughter, Erechtheus killed Eumolpus (original Greek).
Herodotus, Histories 8.44:
These, then, were the Peloponnesians who took part in the war. From the mainland outside the Peloponnese came the following: the Athenians provided more than all the rest, one hundred and eighty ships. They provided these alone, since the Plataeans did not fight with the Athenians at Salamis for this reason: when the Hellenes departed from Artemisium and were off Chalcis, the Plataeans landed on the opposite shore of Boeotia and attended to the removal of their households. In bringing these to safety they were left behind.  The Athenians, while the Pelasgians ruled what is now called Hellas, were Pelasgians, bearing the name of Cranai. When Cecrops was their king they were called Cecropidae, and when Erechtheus succeeded to the rule, they changed their name and became Athenians. When, however, Ion son of Xuthus was commander of the Athenian army, they were called after him Ionians (original Greek).
Pausanias, Description of Greece 1.31.3: