The dialogue you are asking about appears in chapter 5. Gatsby gets Klipspringer out of bed to play the piano for him and Daisy. Klipspringer tells them that he doesn't play very well and tries to say that he hasn't practiced in a long time, but Gatsby won't take no for an answer. After he plays a tune, he turns around on the piano bench and says: “I’m all out of practice, you see. I told you I couldn’t play. I’m all out of prac——."