Musnad Ahmad 31:141

Abu al-Nadr narrated to us, Abu Mu’awiyah—meaning Shayban—narrated to us, from Layth, from ‘Amr ibn Murrah, from Abu al-Bakhtari, from Abu Sa’id, who said: The Messenger of Allah (peace and blessings of Allah be upon him) said: "Hearts are four: A bare heart in which there is something like a lamp that shines; a wrapped heart that is tied within its covering; an inverted heart; and a layered heart. As for the bare heart, it is the heart of the believer; its lamp is its light. As for the wrapped heart, it is the heart of the disbeliever. As for the inverted heart, it is the heart of the hypocrite; he recognized, then denied. As for the layered heart, it is a heart in which there is both faith and hypocrisy. The likeness of faith in it is like a vegetable that is nourished by good water, and the likeness of hypocrisy in it is like a sore that is nourished by pus and blood. Whichever of the two nourishments prevails over the other, it prevails over it."