Musnad Ahmad 32:1447

Yunus narrated to us, Hammad ibn Salamah narrated to us, from Thabit al-Bunani, from Anas ibn Malik, that the Messenger of Allah (peace and blessings of Allah be upon him) said: "When Allah fashioned Adam in Paradise, He left him for as long as Allah willed that He should leave him. Iblis began to circle around him, looking at what he was. When he saw that he was hollow, he knew that he was a creation that would not be able to control itself."