Ezra Pound's advice to young poets includes items that seem to describe Hemingway's later writing:
Use no superfluous word, no adjective which does not reveal something.
Don't be descriptive; remember that the painter can describe a landscape much better than you can, and that he has to know a good deal more about it.
Hemingway wrote poetry early in his career, of course, and kept the poet's ear and eye for language even as he moved on to longer prose works.
Stein's influence may be more subtle, but it might show up in Hemingway's conflicted but intense interest in gender-nonconformity and homosexuality (he writes a lot about those topics, although often with a strong hint of anxiety or disgust) as well as in his repetition of key words and phrases in many of his short stories and novels.