Hawzah News Agency – Majma’ al-Bayan,

وَرَدَتِ الرّوايةُ الصَّحيحة أنّه لمّا نَزَلَت هذهِ الآيَةُ: (فَمَنْ يُرِدِ اللَّهُ أَنْ يَهْدِيَهُ يَشْرَحْ صَدْرَهُ لِلْإِسْلامِ...) سُئلَ رسولُ اللَّهِ صلى اللَّه عليه وآله عن شَرحِ الصَّدرِ ما هُو؟ فقالَ: نورٌ يَقذِفُهُ اللَّهُ في قَلبِ المؤمنِ فَيَنشَرِحُ لَهُ صَدرُهُ ويَنفَسِحُ .
قالوا: فهَل لذلكَ مِن أمارَةٍ يُعرَفُ بها؟ قالَ صلى اللَّه عليه وآله: نَعَم، الإنابَةُ إلى‏ دارِ الخُلودِ، والتَّجافي عن دارِ الغُرورِ، والاستِعدادُ لِلمَوتِ قَبلَ نُزولِ المَوتِ.

It is narrated in a authenticated tradition: When this verse was revealed: "Whomever Allah desires to guide...", the Prophet (SAWA) was asked about the expanding of the breast and what it was. He said, 'It is a light that Allah casts into the heart of a believer and it expands the heart and broadens it.' They asked, 'Is there a sign by which this may be known?' He (SAWA) said, 'Yes, to return to the eternal realm, to shun away from the realm of delusion, and to prepare for death before it comes.'

Source: Majma al-Bayan, v. 4, p. 561