“Each Morn a thousand Roses brings, you say;
Yes, but where leaves the Rose of Yesterday?”

The new pastoral Book.

Mais elle était du monde, où les plus belles choses
Ont le pire destin;
Et Rose, elle a vécu ce que vivent les roses,
L'espace d'un matin.
Letter of condolence to M. Du Perrier on the loss of his daughter, as quoted in Hoyt's New Cyclopedia Of Practical Quotations (1922), p. 680

"Claribel" (1830)
Context: Where Claribel low-lieth
The breezes pause and die,
Letting the rose-leaves fall:
But the solemn oak-tree sigheth,
Thick-leaved, ambrosial,
With an ancient melody
Of an inward agony,
Where Claribel low-lieth.

Eutopia

Source: The Name of the Rose

Source: Dance with the Devil

Book I, line 460, p. 11
The Iliads of Homer, Prince of Poets (1611)

Source: Manuscript Found in Accra

"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)