During a debate with Roger Penrose in 1994 at the Isaac Newton Institute for Mathematical Sciences at the University of Cambridge, transcribed in The Nature of Space and Time (1996) by Stephen Hawking and Roger Penrose, p. 121 http://books.google.com/books?id=LstaQTXP65cC&lpg=PP1&dq=nature%20of%20space%20and%20time&pg=PA121#v=onepage&q=&f=false
Quotes from book
The Nature of Space and Time
The Nature of Space and Time is a book that documents a debate on physics and the philosophy of physics between the British theoretical physicists Roger Penrose and Stephen Hawking. The book was published by Princeton University Press in 1996. The event that is featured in the book took place in 1994 at the University of Cambridge's Isaac Newton Institute. The debate was modeled on the series of debates between Albert Einstein and Niels Bohr.
During the same 1994 exchange with Penrose as the previous quote, transcribed in The Nature of Space and Time (1996) by Stephen Hawking and Roger Penrose, p. 26 http://books.google.com/books?id=LstaQTXP65cC&lpg=PA26&dq=hawking%20%22where%20they%20can't%20be%20seen%22&pg=PA26#v=onepage&q=&f=false and also in "The Nature of Space and Time" (online text) http://arxiv.org/abs/hep-th/9409195
Unsourced variants: Not only does God play dice with the Universe; he sometimes casts them where they can't be seen.
Not only does God play dice, but... he sometimes throws them where they cannot be seen.
Variant: So Einstein was wrong when he said "God does not play dice". Consideration of black holes suggests, not only that God does play dice, but that He sometimes confuses us by throwing them where they can't be seen.