I Am the Walrus

"I Am the Walrus" was, as Jo Grant explained, "a song by the Beatles". The song included the lyrics, "I am he and he is me. And we are all together, goo goo ga goob." Jo brought it up when the Third Doctor unknowingly said part of its lyrics while trying to explain how the Second Doctor was him; the Second Doctor was unfamiliar with it. (TV: The Three Doctors)

Behind the scenes
While the name of song was not actually identified on-screen, the real-world "I Am the Walrus" is a Beatles song with similar lyrics. On 13 November 1972, the copyright holders of the song gave the BBC permission for the lyrics to be quoted.