The text below is taken from the record of the meeting of Stalin and W. Averell Harriman, the U.S. Ambassador to the USSR. Full declassified document is available here. The text was published in Russian by Sergey Oboguev in his blog. Thanks to Sergey for the interesting find.
Conversation. 8:20, Moscow, August 8, 1945.
Present: W. A. Harriman, American Ambassador
George F. Kennan, Minister Counselor
V. M. Molotov, People's Commissar for Foreign Affairs
Mr. Pavlov, Soviet interpreter
The Ambassador asked what he thought of the effects of the news of the atomic bomb would have on the Japanese.
The Generalissimus replied that he thought the Japanese were at present looking for a pretext to replace the present government with one which would be qualified to undertake a surrender. The atomic bomb might give them this pretext.
The Ambassador observed that it was a good thing we had invented this and not the Germans. For long, he said, no one had dared think it would be a success. It was only a few days before the President had told Stalin about it in Berlin that we had learned definitely that it would work succesfully.
The Generalissimus replied that Soviet scientists said that it was a very difficult problem to work out.
The Ambassador said that if the Allies could keep it and apply it for peaceful purposes it would be a great thing.
The Generalissimus agreed and said that would mean the end of the war and aggressors. But the secret would have to be well kept.
The Ambassador said that it could have great importance for peaceful purposes.
The Generalissimus replied, "unquestionably". He added that Soviet scientists had also tried to do it but had not succeeded. They had found one laboratory in Germany where the Germans had evidently been working on the same problem but Russians could not find that they had come to any results. If they had found it, Hitler would never have surrendered. England, too, had gotten nowhere with these researches although they had excellent physicists.
The Ambassador explained that the English had pooled their knowledge with us since 1941. But it had taken enormous installations to conduct the experiments and to achieve final results.
The Generalissimus remarked that this had been very expensive.