“Everything, everything seemed once-upon-a-time.”

Source: The Alchemist (1988), p. 184; this also has been quoted as "What happens once will never happen again. But what happens twice will surely happen a third time."

It seems that this quote has only begun to be attributed to Einstein recently, the earliest published source located being the 2008 book Visualization for Dummies by Bernard Golden, p. 85 http://books.google.com/books?id=2ppZkdmpSlgC&lpg=PP1&pg=PA85#v=onepage&q&f=false. Before that it was often attributed to the physicist John Wheeler, who quoted the saying in Complexity, Entropy, and the Physics of Information, p. 10 http://books.google.com/books?id=mdjsOeTgatsC&lpg=PP1&pg=PA10#v=onepage&q&f=false. In fact, this quip is much older; the earliest source located is Ray Cummings' 1921 short story "The Time Professor", which includes the passage https://books.google.com/books?id=sXpDAQAAMAAJ&pg=PA371#v=onepage&q=%22time%20is%20what%20keeps%20everything%20from%20happening%20at%20once%22&f=false: '"I do know what time is," Tubby declared. He paused. "Time," he added slowly -- "time is what keeps everything from happening at once ...".' Cummings repeated the quote in his 1922 science fiction novel The Girl in the Golden Atom, available on Project Gutenberg here http://www.gutenberg.org/ebooks/21094 (according to Science-Fiction: The Early Years by Everett F. Bleiler, p. 171 http://books.google.com/books?id=KEZxhkG5eikC&lpg=PP1&pg=PA171#v=onepage&q&f=false, the novel was a composite of two earlier stories published in 1919 and 1920). Chapter V http://www.gutenberg.org/files/21094/21094-h/21094-h.htm#CHAPTER_V contains the following paragraph: The Big Business Man smiled. "Time," he said, "is what keeps everything from happening at once." The next-earliest source found for this quote is another book by Ray Cummings, The Man Who Mastered Time http://books.google.com/books?id=YdZEAAAAYAAJ&q=%22everything+from+happening+at+once%22#search_anchor from 1929, and no published examples of the quote from authors other than Cummings can be found until the 1962 Film Facts: Volume 5 where it appears on p. 48 http://books.google.com/books?id=sr0vAQAAIAAJ&q=%22everything+from+happening+at+once%22#search_anchor. So, it seems likely that Ray Cummings is the real originator of this saying.
Misattributed

Wheeler quoted this saying in Complexity, Entropy, and the Physics of Information (1990), p. 10 http://books.google.com/books?id=mdjsOeTgatsC&lpg=PP1&pg=PA10#v=onepage&q&f=false, with a footnote attributing it to "graffiti in the men's room of the Pecan Street Cafe, Austin, Texas". Later publications, such as Paul Davies' 1995 book About Time (p. 236), credited Wheeler with variations of this saying, but the quip is actually much older. The earliest known source is Ray Cummings' 1922 science fiction novel The Girl in the Golden Atom, http://www.gutenberg.org/ebooks/21094 Ch. V: " 'Time,' he said, 'is what keeps everything from happening at once.' " It also appears in his 1929 novel The Man Who Mastered Time. http://books.google.com/books?id=YdZEAAAAYAAJ&q=%22everything+from+happening+at+once%22#search_anchor The earliest known occurrence other than Cummings is from 1962 in Film Facts: Volume 5, p. 48 http://books.google.com/books?id=sr0vAQAAIAAJ&q=%22everything+from+happening+at+once%22.
Misattributed

Una cosa, hasta no ser toda, es ruido, y todo, es silencio.
Voces (1943)

MOJO interview (2005)
Context: There were so many times I thought, "I'll have the album finished this year, definitely, we'll get it out this year." Then there were a couple of years where I thought, "I'm never gonna do this." If I could make albums quicker, I'd be on a roll wouldn't I? Everything just seems to take so much time. I don't know why. Time… evaporates.

"Sometimes", § 5
Red Bird (2008)
Context: Two or three times in my life I discovered love.
Each time it seemed to solve everything.
Each time it solved a great many things
but not everything.
Yet left me as grateful as if it had indeed, and
thoroughly, solved everything.

Source: It's Not Summer Without You

14 April 1747
Letters to His Son on the Art of Becoming a Man of the World and a Gentleman (1774)