זוהר שמות — דף קי ע”ב

Moses received seventy keys of the Torah, and the seventieth, hidden and called Eitan, was not given to him. The Master told Moses that its name is Eitan, and that all upper and lower keys depend on this secret. The Tannaim are the Eitanim of the Oral Torah outside the body, and the Mashkil le-Eitanim is the supernal brain that instructs David. Tishri bears this name because the Holy One sealed within it the letter yod of His name, and Devorah sealed the vav.