Source, in full
As discussed in this entry from The Quote Investigator, the earliest published attribution of a similar quote to Einstein seems to have been in Gestalt therapist Frederick S. Perls' 1969 book Gestalt Theory Verbatim, where he wrote on p. 33: "As Albert Einstein once said to me: 'Two things are infinite: the universe and human stupidity.' But what is much more widespread than the actual stupidity is the playing stupid, turning off your ear, not listening, not seeing." Perls also offered another variant in his 1972 book In and Out the Garbage Pail, where he mentioned a meeting with Einstein and on p. 52 quoted him saying: "Two things are infinite, the universe and human stupidity, and I am not yet completely sure about the universe." However, Perls had given yet another variant of this quote in an earlier book, Ego, Hunger, and Aggression: a Revision of Freud's Theory and Method (originally published 1942, although the Quote Investigator only checked that the quote appeared in the 1947 e
Variants
"Only two things are infinite, the universe and human stupidity, and I'm not sure about the former." Earliest version located is in Technocracy digest: Issues 287–314 from 1988, p. 76. Translated to German as: "Zwei Dinge sind unendlich: das Universum und die menschliche Dummheit. Aber beim Universum bin ich mir nicht ganz sicher." (Earliest version located - "Zwei Dinge sind unendlich, das Universum und die menschliche Dummheit . . . Und beim Universum bin ich mir noch keineswegs sicher" - in Hans Askenasy: Sind wir alle Nazis? Zum Potential der Unmenschlichkeit, Campus Verlag Frankfurt/Main 1979, p. 153 books.google.)