Musnad Ahmad 32:1572

Hassan and ‘Affan narrated to us—the meaning being the same—they said: Hammad ibn Salamah narrated to us, Thabit al-Bunani informed us, from Anas ibn Malik, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "When Allah fashioned Adam, He left him for as long as Allah willed that He should leave him. Then 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 itself."