Allah Ta’ala says in the Qur’ân:
إِلَّا مَنْ أَتَى اللَّهَ بِقَلْبٍ سَلِيمٍ
“But only he (will prosper) that brings to Allah Ta’ala a sound heart”. (26:89)
For real prosperity you need a properly functioning heart that is liked by Allah Ta’ala. Saleem is a word derived from Salaamat. It is a broad term but we can translate it as 'safe', also. Qalb is necessary for the real prosperity and safe Qalb is liked by Allah Ta’ala.
The safe Qalb means that all the qualities (latent or manifest) that have been given to it, are not wasted. The immense potential, which Allah Ta’ala has bestowed to the human Qalb is intact and well used and not disused.
It was already mentioned that man has been selected for expression of all the Attributes of Allah Ta’ala. Man's Qalb is framed to accommodate the faculties of quite different and opposing nature, as man is a mixture of opposites. If each faculty of Qalb remains in its right proportion and limits and each does its job in a just manner, it can be called as Saleem (safe). What is safe, is sound and healthy as well. If it is not safe, it is not sound and healthy i.e., it is diseased. When it is diseased, it stops doing its function. It is the king of all parts of the body and when the king becomes defunct one can imagine the fate of the nation (the man).
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)
When the king is in right condition and functions properly, the subjects will be in right condition and they will also function properly and vice versa.