Bruce Lee's instruction to abandon fixed form and take the shape of whatever contains you.
Documented
The Longstreet script
The line aired in a 1971 episode of the TV series Longstreet, written by Stirling Silliphant after interviews with Lee. Lee plays an instructor telling a blind detective to empty his mind and be formless, shapeless, like water. The wording was scripted television refined from Lee's real teaching notes.
Assessment: Documented: the broadcast and the screenplay are the primary artifacts.
Stories
A street fighter's summary
Lee retold the idea as the lesson of his whole life: a hot-headed teenager in Hong Kong, calmed by Ip Man's wing chun, learning that softness beats rigidity. Fans read the water speech as autobiography compressed into metaphor.
Assessment: Plausible: the metaphor matches his training history, but the neat origin story is polished hindsight.
Interesting claims
A two-thousand-year echo
The image is older than Lee by millennia: the Dao De Jing praises water as the softest thing that overcomes the hardest. Some hear the Longstreet scene as Laozi translated into a television close-up, knowingly or not.
Assessment: Interesting: the parallel is real in substance, the line of transmission is the only question.
Sources and further reading
Longstreet, 'The Way of the Intercepting Fist' (1971), ABC television episode, screenplay by Stirling Silliphant
Bruce Lee, Tao of Jeet Kune Do (1973), edited by Gilbert Johnson
Bruce Lee Podcast, episode discussions of the Longstreet script, brucelee.com
Worth asking
If water always wins, why does anyone still build walls?