Louis Ginzberg , The Legends of the Jews, , [0], vol 3., The First Commandment
The first word of God on Sinai was Anoki, "It is I." It was not a Hebrew word, but and Egyptian word that Israel first heard from God. He treated them as did that king his home-coming son, whom, returning from a long stay over sea, he addressed in the language the son had acquired in a foreign land. So God addressed Israel in Egyptian, because it was the language they spoke. At the same time Israel recognized in this word "Anoki," that is was God who addressed them. For when Jacob had assembled his children around his death-bed, he warned them to be mindful of the glory of God, and confided to them the secrets that God would hereafter reveal to them with the word "Anoki." He said: "With the word 'Anoki' He addressed my grandfather Abraham; with the word 'Anoki' He addressed my father Isaac, and with the word 'Anoki' He addressed me. Know, then, that when He will come to you, and will so address, you, it will be He, but not otherwise."