Rasulullah (Sallallahu Alaihi Wasallam) says:
ألا وإنّ في الجَسَدِ مُضْغَةً إذا صَلَحَتْ صَلَحَ الجَسدُ كُلُّهُ وإذا فَسَدَتْ فَسَدَ الجَسَدُ كُلُّهُ ألا وهْيَ القَلبُ

" Indeed there is a piece of flesh in the body (the heart) when that stays right, the whole body stays right, when that goes astray, the whole body goes astray! Listen with open ears, that is Qalb" (Bukhâri Sharief)