From the New York Times obituary of Rudolf Serkin :
"Among the dozens of recordings he made, those in which he teamed as a chamber-music partner with Adolf Busch, the German violinist, are especially prized by collectors. It was Mr. Busch who promoted the young pianist's European career, presenting him as a soloist in Bach's Brandenburg Concerto No. 5 at Mr. Serkin's Berlin debut in 1921.
Mr. Serkin, recalling this in the magazine Clavier, said: "I was 17 years old. At tne end of the concert, because it had been a great success, Busch pushed me out, saying I should play an encore. 'What shall I play?' I asked. 'The Goldberg Variations,' he replied, as a joke. (Without repeats, the Bach work takes half an hour to perform.) And I took him seriously. When I finished there were only four people left: Adolf Busch, Arthur Schnabel, Alfred Einstein [ the musicologist ] and me."