بسم الله الرحمن الرحيم
Ibn Al-Qayyim – rahimahu Allah – said:
Likewise, from among them [i.e. the causes that alleviate the chest] is the light that Allah casts in to the heart of the slave. It is the light of imaan, for indeed it alleviates and widens the chest and brings joy to the heart.
So if this light is lost from the heart of the slave, it [i.e. his heart] becomes narrow and uneasy. He [i.e. the slave] ends up in the most constricted and difficult of prisons.
Indeed At-Tirmidhi narrated in his Al-Jaami’, from the Prophet ﷺ that he said:
“إذا دَخَلَ النور القلبَ، انْفَسَحَ وانشرحَ». قالوا: وما عَلاَمَةُ ذَلِكَ يَا رسُولَ اللَّهِ؟ قال: «الإنَابَةُ إلى دارِ الخُلُودِ، والتَجَافِي عَنْ دَارِ الغُرُورِ، والاسْتِعْدادُ للمَوْتِ قَبْلَ نُزوله”.
“If light enters the heart, it [i.e. the heart] widens and becomes alleviated”. They said: “And what is the sign of that O Allah’s Messenger”? He said: “The [desire for] returning to the abode of eternity, forsaking the abode of delusion and preparing for death before its arrival”.
So the slave receives that degree of alleviation to his chest in accordance to the degree of light that he receives. Likewise, concerning perceivable light and perceivable darkness; the former alleviates the chest and the latter constricts it.
Source: Ibn Al-Qayyim, Zaad Al-Ma’aad, Chapter: Concerning the causes of alleviating the chest, [of which] complete and perfect acquisition belongs to him [i.e. the Messenger] ﷺ