“Rose is a rose is a rose is a rose.”

"Sacred Emily"
This statement, written in 1913 and first published in Geography and Plays, is thought to have originally been inspired by the work of the artist Sir Francis Rose; a painting of his was in her Paris drawing-room.
See also the Wikipedia article: Rose is a rose is a rose is a rose
Nigel Rees explains the phrase thus: "The poem 'Sacred Emily' by Gertrude Stein (1874-1946) is well-nigh impenetrable to the average reader but somehow it has managed to give a format phrase to the language. If something is incapable of explanation, one says, for example, 'a cloud is a cloud is a cloud.' What Stein wrote, however, is frequently misunderstood. She did not say 'A rose is a rose is a rose,' as she might well have done, but 'Rose is a rose is a rose is a rose' (i.e. no indefinite article at the start and three not two repetitions.) The Rose in question was not a flower but an allusion to the English painter, Sir Francis Rose, 'whom she and I regarded' wrote Constantine Fitzgibbon, 'as the peer of Matisse and Picasso, and whose paintings — or at least painting — hung in her Paris drawing-room while a Gauguin was relegated to the lavatory.'" - Sayings of the Century, page 91
Geography and Plays (1922)