“The word must be heard in silence; there must be darkness to see the stars.”

Comprehension
One Minute Wisdom (1989)

Source: Earthsea Books, The Farthest Shore (1972), Chapter 9, "Orm Embar" (Sparrowhawk)

The Cherubinic Wanderer

Source: Dictionary of Burning Words of Brilliant Writers (1895), P. 590.

The Sparkling Gem, complete works, vol. 3, pp. 6-7

Source: Earthsea Books, A Wizard of Earthsea (1968), Chapter 9

Misattributed
Variant: When its dark enough you can see the stars.

Widely attributed to Ralph Waldo Emerson on the internet; however, a presumably definitive source of Emerson's works at http://www.rwe.org fails to confirm any occurrence of this phrase across his works. This phrase is found in remarks attributed to Charles A. Beard in Arthur H. Secord, "Condensed History Lesson", Readers' Digest, February 1941, p. 20; but the origin has not been determined. Possibly confused with a passage in "Illusions" in which Emerson discusses his experience in the "Star Chamber": "our lamps were taken from us by the guide, and extinguished or put aside, and, on looking upwards, I saw or seemed to see the night heaven thick with stars glimmering more or less brightly over our heads, and even what seemed a comet flaming among them. All the party were touched with astonishment and pleasure. Our musical friends sung with much feeling a pretty song, “The stars are in the quiet sky,” &c., and I sat down on the rocky floor to enjoy the serene picture. Some crystal specks in the black ceiling high overhead, reflecting the light of a half–hid lamp, yielded this magnificent effect."
Misattributed

Source: Living in Truth (1986), The Power of the Powerless