Musnad Ahmad 32:1717

Affan narrated to us, Hammad narrated to us, Thabit informed us, from Anas, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When Allah created Adam, He fashioned him, then He left him in Paradise for as long as Allah willed to leave him. So Iblis began to circle around him. When he saw that he was hollow, he knew that he was a creation that would not be able to control himself."