“When the new wore off his grandfather’s watch, and carrying it became a day’s burdensome task, Jem no longer felt the necessity of ascertaining the hour every five minutes.”

Source: The Beloved Returns (1939), Ch. 7
Context: Hold fast the time! Guard it, watch over it, every hour, every minute! Unregarded it slips away, like a lizard, smooth, slippery, faithless, a pixy wife. Hold every moment sacred. Give each clarity and meaning, each the weight of thine awareness, each its true and due fulfillment.

The Glass Bead Game (1943)
Context: It is a pity that you students aren't fully aware of the luxury and abundance in which you live. But I was exactly the same when I was still a student. We study and work, don't waste much time, and think we may rightly call ourselves industrious — but we are scarcely conscious of all we could do, all that we might make of our freedom. Then we suddenly receive a call from the hierarchy, we are needed, are given a teaching assignment, a mission, a post, and from then on move up to a higher one, and unexpectedly find ourselves caught in a network of duties that tightens the more we try to move inside it. All the tasks are in themselves small, but each one has to be carried out at its proper hour, and the day has far more tasks than hours. That is well; one would not want it to be different. But if we ever think, between classroom, archives, secretariat, consulting room, meetings, and official journeys — if we ever think of the freedom we possessed and have lost, the freedom for self-chosen tasks, for unlimited, far-flung studies, we may well feel the greatest yearning for those days, and imagine that if we ever had such freedom again we would fully enjoy its pleasures and potentialities.

An explanation of relativity which he gave to his secretary Helen Dukas to convey to non-scientists and reporters, as quoted in Best Quotes of '54, '55, l56 (1957) by James B. Simpson; also in Expandable Quotable Einstein (2005) edited by Alice Calaprice

William Hermanns recorded a series of four conversations he had with Einstein and published them in his book Einstein and the Poet (1983), quoting Einstein saying this variant in a 1948 conversation: "To simplify the concept of relativity, I always use the following example: if you sit with a girl on a garden bench and the moon is shining, then for you the hour will be a minute. However, if you sit on a hot stove, the minute will be an hour." ( p. 87 http://books.google.com/books?id=QXCyjj6T5ZUC&lpg=PP1&pg=PA87#v=onepage&q&f=false)

In the 1985 book Einstein in America, Jamie Sayen wrote "Einstein devised the following explanation for her [Helen Dukas] to give when asked to explain relativity: An hour sitting with a pretty girl on a park bench passes like a minute, but a minute sitting on a hot stove seems like an hour." ( p. 130 http://books.google.com/books?ei=yma3TsDWK8WciQL63smAAQ&ct=book-thumbnail&id=vs3aAAAAMAAJ&dq=sayen+%22einstein+in+america%22&q=pretty+girl#search_anchor)
Attributed in posthumous publications
Variant: When you are courting a nice girl an hour seems like a second. When you sit on a red-hot cinder a second seems like an hour. That's relativity.

"The transition to democracy was based on a base of harmony and little remembrance." (It was good that the Transition) "[...] was like that, because at the time the wounds were still open. That problem affected an entire generation of Spaniards. However, the generation that I come from is drawn to politics in a context of democracy and liberty, and now it's only fair that the sacrifices that many people made are recognised and that people know exactly what happened to their relatives, because it's their right [...] this right doesn't involve looking back with a grudge, but completely the opposite: it means looking back with serenity, to find out the truth [...] it means building a stronger country, a country that can look at all of its citizens with absolute serenity, so that they feel recognised in our project of contemporary democracy".
Interview in 2005 in the book "Zapatero and the Citizens' World" by Calamai and Garzia.
As President, 2005

Source: The Realm of Possibility

Book II, ch. 4 (trans. Constance Garnett)
The Elder Zossima, speaking to Mrs. Khoklakov
The Brothers Karamazov (1879–1880)

The Roycraft Dictionary and Book of Epigrams (1923)
Source: The Roycroft Dictionary Concocted by Ali Baba and the Bunch on Rainy Days

Talks at Google (Oct 28, 2014)

There is no indication that Einstein said this. According to Quote Investigator, the earliest publication of a quote similar was in a collection of articles about manufacturing in 1966, when an employee of the Stainless Processing Company wrote a piece titled "The Manufacturing Manager's Skills." The article attributed the quote to an unnamed professor at Yale, by saying, "If I had only one hour to solve a problem, I would spend up to two-thirds of that hour in attempting to define what the problem is." (See, 1966, The Manufacturing Man and His Job by Robert E. Finley and Henry R. Ziobro, "The Manufacturing Manager's Skills" by William H. Markle (Vice President, Stainless Processing Company, Chicago, Illinois), Start Page 15, Quote Page 18, Published by American Management Association, Inc., New York. Verified on paper). https://quoteinvestigator.com/2014/05/22/solve/
Disputed
Variant: If I had an hour to solve a problem I'd spend 55 minutes thinking about the problem and 5 minutes thinking about solutions.