Imam Ahmad recorded Abu Sa`id saying that the Messenger of Allah Sallallahu alihiwassalam said: The hearts are four (types): polished as shiny as the radiating lamp, a sealed heart with a knot tied around its seal, a heart that is turned upside down and a wrapped heart. As for the polished heart, it is the heart of the believer and the lamp is the light of faith. The sealed heart is the heart of the disbeliever. The heart that is turned upside down is the heart of the pure hypocrite, because he had knowledge but denied it. As for the wrapped heart, it is a heart that contains belief and hypocrisy. The example of faith in this heart, is the example of the herb that is sustained by pure water. The example of hypocrisy in it, is the example of an ulcer that thrives on puss and blood. Whichever of the two substances has the upper hand, it will have the upper hand on that heart.
SubhanAllah, I have nothing to add to this. This hadeeth is so clear even though very eloquent and filled with parables. InshAllah we should try to be the possessor of the heart that is polished as shiny as the radiating lamp. And we should not allow ulcer to grow in us. Wouldn't all of us love to STAY HEALTHY AND DISEASE-FREE?!