I had written to Einstein to learn what he thought of such a plan. He had replied with enthusiasm, but had said that, because he was not well and could hardly keep up with present commitments, he himself could do nothing to help beyond sending me the names of various scientists who, he thought, would be sympathetic. He had begged me, nevertheless, to carry out my idea and to formulate the statement (= Russell-Einstein Manifesto) myself. This I had done, basing the statement upon my Christmas broadcast, 'Man's Peril'.
Source: The Autobiography of Bertrand Russell, v.3 chap. 2: At home and abroad, 1969
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