Musnad Abu Ya'la al-Mawsili 148:51

Muhammad ibn Bahr narrated to us, 'Adi ibn Abi 'Amarah narrated to us, Ziyad al-Numayri narrated to us, from Anas ibn Malik, who said: The Messenger of Allah (may Allah bless him and grant him peace) said: "Indeed, the Shaytan places his snout upon the heart of the son of Adam. If he remembers Allah, he withdraws; and if he forgets, he swallows his heart. That is the 'whispering-sneaking' (al-waswas al-khannas)."