As-Salaam alaikum,
In ayat 22 of Surat Zumar of the Holy Qur'an, Allah Ta'ala says:--

"Is one whose heart Allah has opened to Islam, so that
he has received enlightenment from Allah, (no better
than one hard-hearted)?
Woe to those whose hearts are hardened against celebrating the Praises of Allah!
They are manifestly wandering (in error)!" (39:22)

The Messenger of Allah, Sallallahu alaihi Wasallam, was asked about sharh as-sadr and he said, "It is a light cast into the heart by which the chest is opened and expanded."

He was asked, "Is there a way of recognizing one who possesses that light?" He, alaihi salaat was-salaam, responded:
"Yes, [such people can be recognized by their] indifference [about] the abode of delusions [ie, the life of this world], their constant turning towards the abode of eternity [the hereafter], and their preparation for death before it arrives."

He was asked, "How does one prepare for death"?
He, Sallallahu alaihi Wasallam, said: "Those of you who remember it the most are the best prepared for it."

Source: 'Risalat Sharh as-Sadr', by Imam Abu Hamid al-Ghazali.