“Those who dream by day are cognizant of many things which escape those who dream only by night.”

Introductory Chapter. Variant: This, therefore, is a faded dream of the time when I went down into the dust and noise of the Eastern market-place, and with my brain and muscles, with sweat and constant thinking, made others see my visions coming true. Those who dream by night in the dusty recesses of their minds wake in the day to find that all was vanity; but the dreamers of the day are dangerous men, for they may act their dream with open eyes, and make it possible.
Seven Pillars of Wisdom (1922)

SPIEGEL Interview with Daniel Barenboim

Moving Waters Exhibit Gallery 12 http://www.movingwaters.org/exhibitgallery/exhibit12.html

Source: Dracula

Source: Poems of Fernando Pessoa

Often attributed to Eleanor Roosevelt without an original source in her writings, for example in the introduction to It Seems to Me : Selected Letters of Eleanor Roosevelt (2001) by Leonard C. Schlup and Donald W. Whisenhunt, p. 2 http://books.google.com/books?id=UeFWjTMcLZYC&lpg=PP1&pg=PA2#v=onepage&q&f=false. But archivists have not been able to find the quote in any of her writings, see the comment from Ralph Keyes in The Quote Verifier above.
Disputed

Perhaps confusion of Book VI, Chapter II, 30
Similar to Matthew Prior: "For hope is but the dream of those that wake", Solomon on the Vanity of the World, book iii, line 102.
Misattributed