“It will never rain roses: when we want to have more roses, we must plant more roses.”

"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)

Song Roses of Picardy http://www.firstworldwar.com/audio/rosesofpicardy.htm

The new pastoral Book.

Section 7 : Spiritual Progress
Life and Destiny (1913)

Je ne suis pas la rose, mais j’ai vécu avec elle.
A. Hayward, Autobiography and Letters of Mrs. Piozzi, Introduction.

A Rose to the Living, reported in Bartlett's Familiar Quotations, 10th ed. (1919).

The "Camelot" interview (29 November 1963)