“Every now and then a man's mind is stretched by a new idea or sensation, and never shrinks back to its former dimensions. After looking at the Alps, I felt that my mind had been stretched beyond the limits of its elasticity, and fitted so loosely on my old ideas of space that I had to spread these to fit it.”

Also reported as "One's mind" instead of "A man's mind", and "can never go back" or "never regains" instead of "never goes back"; most likely properly attributed to Oliver Wendell Holmes, Sr.
Misattributed

Epilogue
Novels, Squeeze Me (2020)

Source: Think Big (1996), p. 224

Source: The Mind Parasites (1967), p. 38

The body is a mere wall between two infinities. Space extends to infinity outwards; the mind stretches to infinity inwards.
Source: The Mind Parasites (1967), p. 38

Source: The Complete Poems of Emily Dickinson

Actually said by Oliver Wendell Holmes, Sr. in his book The Autocrat of the Breakfast Table: "Every now and then a man's mind is stretched by a new idea or sensation, and never shrinks back to its former dimensions."
Misattributed

C. K. Prahalad, cited in: Don Soderquist (2005), The Wal-Mart Way, p. 178