As quoted by Alexander Moszkowski (Einstein: Einblicke in seine Gedankenwelt, 1921, p. 37), with the chapter in which this text appears dated 29th March, 1920:
Es existiert vorläufig nicht der leiseste Anhalt dafür, ob und wann jemals diese Energiegewinnung erzielt werden könnte." Denn sie würde einen „erzwungenen Atomzerfall", eine von Menschen bewirkte „Atomzermalmung" voraussetzen, und für diese Möglichkeit liegt bis heute auch nicht das leiseste Anzeichen vor. Den Atomzerfall können wir nur beobachten, wo die Natur selbst ihn uns darbietet, wie beim Radium, dessen Aktivität auf dem dauernden, explosiven Zerfall seiner Atome beruht. „Allein, wir können diesen Vorgang nur feststellen, nicht hervorrufen, und bei dem heutigen Stand der Wissenschaft erscheint es so gut wie ausgeschlossen, daß wir dazu jemals gelangen könnten.
Translation by Henry L. Brose (Einstein the Searcher: His Work Explained from Dialogues with Einstein, 1922, p. 24):
At present there is not the slightest indication of when this energy will be obtainable, or whether it will be obtainable at all. For it would presuppose a disintegration of the atom effected at will—a shattering of the atom. And up to the present there is scarcely a sign that this will be possible. We observe atomic disintegration only where Nature herself presents it, as in the case of radium, the activity of which depends upon the continual explosive decomposition of its atom. Nevertheless, we can only establish the presence of this process, but cannot produce it; Science in its present state makes it appear almost impossible that we shall ever succeed in so doing.
From "Available Energy" (Messel Memorial Lecture, 4th September, 1928) [DOI: 10.1002/jctb.5000474013]:
Under the stimulus of the discovery of the enormous quantities of energy evolved in the disintegration of uranium and thorium we have often imagined, and sometimes incautiously stated, that there might be similar amounts of available energy locked up in the common elements, releasable, perchance, by getting them to disintegrate as uranium and thorium spontaneously are doing. And engineers, physicists, and laymen alike have talked glibly about "utilising this source of energy when the coal is gone." So-called humanists, on the other hand, advocates of a return to the "glories" of a pre-scientific age, have pictured the diabolical scientist tinkering heedlessly, like the bad small boy, with these enormous stores of sub-atomic energy and some sad day touching off the fuse and blowing our comfortable little globe to smithereens.
But Nature, or God, whichever term you prefer, was not unconscious of the wisdom of introducing a few fool-proof features into the machine. If Einstein's equation and Aston's curve are even roughly correct, as I am sure they are, for Dr. Cameron and I have computed with their aid the maximum energy evolved in radioactive change and found it to check well with observation, then this supposition of an energy evolution through the disintegration of the common elements is from the one point of view a childish Utopian dream, and from the other a foolish bugaboo. For the great majority of the elements such as constitute the bulk of our world are in their state of maximum stability already. They have no energy to give up in the disintegrating process. They can only be broken apart by working upon them, or by supplying energy to them. Man can probably learn to disintegrate them, but he will always do it "by the sweat of his brow."
The energy available to him through the disintegration of radioactive, or any other, atoms may perhaps be sufficient to keep the corner peanut and pop-corn man going on a few street corners in our larger towns for a long time yet to come, but that is all.
The energy available to him through the building up of the common elements out of the enormous quantities of hydrogen existing in the waters of the earth would be practically unlimited provided such atom-building processes could be made to take place on the earth. But the indications of the cosmic rays are that these atom-building processes can take place only under the conditions of temperature and pressure existing in interstellar space. Hence there is not even a remote likelihood that man can ever tap this source of energy at all. The hydrogen of the oceans is not likely ever to be converted by man into helium, oxygen, silicon, or iron.
The energy supplied to man in the past has been obtained wholly from the sun, and a billion years hence he will still, I think, be supplying all his needs for light and warmth and power entirely from the same source. ...
When the matter of the sun has all been stoked into his furnaces, and they are gone completely out, another sun will probably have been formed so that on this earth or on some other earth, it matters not which, a few billion years hence, the development of man may still be going on.
1933-09-11 address at Leicester to the British Association for the Advancement of Science (quoted in New York Herald Tribune, "Atom-Powered World Absurd, Scientists Told: Lord Rutherford Scoffs at Theory of Harnessing Energy in Laboratories", 1933-09-12, through The Associated Press, reproduced in History of Physics, 1985, p. 117):
The energy produced by the breaking down of the atom is a very poor kind of thing. Any one who expects a source of power from the transformation of these atoms is talking moonshine. ... We hope in the next few years to get some idea of what these atoms are, how they are made and the way they are worked.
But see Jenkin (2011) who argues that:
Ernest Rutherford, in all of his later negative pronouncements regarding the possibility of atomic energy, was adopting a quite deliberate policy to disguise and postpone, for as long as possible, the awful prospect that he saw looming over the horizon: a new and dreadful war, a new and devastating weapon, and unprecedented destruction.