Herodotus
On this lake it is that the Egyptians represent by night his sufferings whose name I refrain from mentioning, and this representation they call their Mysteries. I know well the whole course of the proceedings in these ceremonies, but they shall not pass my lips. So too, with regard to the mysteries of Demeter, which the Greek term “the Thesmophoria,” I know them, but I shall not mention them, except so far as may be done without impiety. The daughters of Danaus brought these rites from Egypt, and taught them to the Pelasgic women of the Peloponnese. Afterward, when the inhabitants of the peninsula were driven from their homes by the Dorians, the rites perished. Only in Arcadia, where the natives remained and were not compelled to migrate, their observance continued. – 2.171
The Egyptians say that Demeter and Dionysos are the rulers of the lower world. The Egyptians were the first who maintained the following doctrine, too, that the human soul is immortal, and at the death of the body enters into some other living thing … There are Greeks who have used this doctrine [the Orphics]. – 2.123
Apollon and Artemis were children of Dionysos and Isis, and Leto was made their nurse and preserver; in Egyptian, Apollon is Horus, Demeter Isis, Artemis Bubastis. – 2.156