Is it possible that what is wanted is the reference to the passage in which Dill runs away from home and sneaks into Scout's bedroom and hides under her bed? When she passes near the bed she feels "something warm, resilient, and rather smooth." Running back to Jem's room, she knocks and asks what a snake feels like. Jem procures a broom and makes "a tentative swipe under the bed." They discover Dill.
In a show of maturity against keeping quiet as Scout is tempted to do, Jem tells Dill that he should let his mother know where he is since he has been under the bed for two hours. Dill looks at him and Jem "rose and broke the remaining code of our childhood." He calls Atticus and reveals Dill. This all occurs in Chapter 14 of To Kill a Mockingbird.
Having read the novel, To Kill a Mockingbird, many times before, I could not place a scene where these characters lie down together at any point. After carefully searching the chapters once again, I still found no evidence of this. Dill and Scout do share a bed innocently enough in Chapter 14 after he has run away from home and shown up under her bed, but Atticus does not join them. Dill does tell many lies during the story, but he is pretty straight forward with Atticus, except for when he lies about Jem losing his pants while playing strip poker.