That lyric is from "Tomorrow Never Knows.

Yes. “Turn off your mind, relax and float downstream” is the opening line of “Tomorrow Never Knows,” the final track on Revolver (1966).

It’s not your typical Beatles song. John Lennon wrote it after reading The Psychedelic Experience (a take on the Tibetan Book of the Dead), and the band went full experimental: tape loops, reversed guitars, Ringo’s hypnotic drum pattern, and Lennon singing through a spinning Leslie speaker cabinet.

The whole thing sounds like a mantra beamed from another planet. It’s also famous for being one of the first pop songs to use sampling (the seagull-sounding loop is a sped-up recording of Paul laughing). If you’ve never listened to it with headphones, do that. It holds up.