Chasing this quote all over the internet seems to indicate an involved history. Various quote-sites give it as "El poema, la canción, la imagen, son solo agua extraída del pozo de la gente, y se les debe devolver en una copa de belleza para que puedan beber, y comprendan ellos mismos." But not a one indicates where/when Lorca wrote this, and there are a few variations. I did, however, find the following passage in Langston Hughes's autobiography; he was visiting Spain, and talking to Rafael Alberti, the Secretary General of the Alliance of Anti-Fascist Intellectuals, about their latest play. He reports that Alberti said to him, "What the members of the Alianza want to do is make art life, and life art, with no gulf between the artists and the people. After all, as Lorca said, 'The poem, the song, the picture is only water drawn from the well of the people, and it should be given back to them in a cup of beauty so that they may drink--and in drinking, understand themselves.'" Was this quote, then, ever even written down by Lorca, or only passed on by his fellow poet Alberti? Alberti probably said it to other people than Hughes; oral transmission would account for variation.