Tafsir of Al-Mutaffifeen 83:14

Nay! But on their hearts is the Ran (covering) which they used to earn. meaning, the matter is not as they claim, nor as they say: "Verily, this Qur'an is tales of the ancients.'' Rather, it is the Word of Allah, His inspiration and His revelation to His Messenger. The only thing that blocked their hearts from believing in it is the dark covering cast over it from the many sins and wrong they committed that has covered up their hearts. Thus, Allah says,

كﻼ بل ران على قلوبهم ما كانوا يكسبون

(Nay! But on their hearts is the Ran (covering) which they used to earn.) - This dark covering known as Rayn overcomes the hearts of the disbelievers, - the covering of Ghaym is for the righteous, and - the covering of Ghayn is for those who are near to Allah. Ibn Jarir, At-Tirmidhi, An-Nisa'i, and Ibn Majah all recorded from Abu Hurayrah that the Prophet said,

إن العبد إذا أذنب ذنبا كانت نكتة سوداء في قلبه فٳن تاب منها صقل قلبه قوإن زاد زادت فذلك قول الله تعالى

كﻼ بل ران على قلوبهم ما كانوا يكسبون

Verily, when the servant commits a sin, a black spot appears in his heart. If he repents from it, his heart is polished clean. However, if he increases (in the sin), the spot will continue to increase. That is the statement of Allah: (Nay! But on their hearts is the Ran (covering) which they used to earn.) At-Tirmidhi said, "Hasan Sahih.'' The wording of An-Nasa'i says,

إن العبد إذا أخطٲ خطيية نكت في قلبه نكتة سوداء فٳن هو نزع واستغفر وتاب صقل قلبه فٳن عاد زيد فيها حتى تعلو قلبه فهو الران الذي قال الله تعالى

كﻼ بل ران على قلوبهم ما كانوا يكسبون

Whenever the servant commits a wrong, a black spot is put in his heart. So, if he refrains from it, seeks forgiveness and repents, his heart is polished clean. But if he returns to the sin, the spot will increase until it overcomes his (entire) heart, and this is the Ran that Allah mentions when He says: (Nay, but on their hearts is the Ran (covering) which they used to earn.) Concerning Allah's statement,

كﻼ إنهم عن ربهم يوميذ لمحجوبون