Hazrat Moulana Ashraf Ali Thanwi (rahmatullahi ‘alaih) once mentioned the following:
When a person is punctual upon performing his five daily salaah, then he acquires a special noor (light) in his heart, and the effect of this noor then becomes visible on his face.On the contrary, when a person neglects performing his five daily salaah, then his heart is filled with darkness, and the effect of this darkness is then visible on his face.In actual fact, when one neglects performing salaah, then it is as if there is a fire that is burning within him. The smoke of this burning fire affects his inner and outer condition (i.e. his heart and face) as it causes darkness to cover both.The result of the heart being blackened is that it no longer is able to perceive any feeling of disgust or detest for sins such as bribery, speaking lies, slandering people, stealing people’s land and property, taking loans and thereafter denying the loan to avoid repayment, casting lustful glances at women and young lads, adopting the culture and ways of the Christians (Jews and other kuffaar) and many other such evils.
(Malfoozaat Hakeemul Ummat 23/121)