Verily in the Remembrance of Allah do the hearts find rest
Thus, Allāh (The Exalted) says:
"Those who believed (in the Oneness of Allāh - Islamic Monotheism), and whose hearts find rest in the remembrance of Allāh: Verily, in the remembrance of Allāh do the hearts find rest."
So, restfulness of hearts is to be happy and tranquil, free from worries and annoyance; this can be achieved by the remembrance of Allāh. Without the remembrance of Allāh, the heart will die. Abu Musa Al-Asha’ri narrated that the Prophet (May Allāh raise his rank and grant him peace) said:
مَثَلُ الَّذِي يَذْكُرُ رَبَّهُ وَ الَّذِي لَا يَذْكُرُ رَبَّهُ مَثَلُ الْحَيِّ وَ الْمَيِّتِ
"The similitude of one who remembers his Lord and one who does not remember Him is like that of the living and the dead."
The Prophet (May Allāh raise his rank and grant him peace) gave the example of the one who remembers Allāh with the living and the one who does not remember Allāh with the dead; and in another narration by Muslim, the Prophet (May Allāh raise his rank and grant him peace) said:
مَثَلُ الْبَيْتِ الَّذِي يُذْكَرُ اللَّهُ فِيهِ وَالْبَيْتِ الَّذِي لَا يُذْكَرُ اللَّهُ فِيهِ مَثَلُ الْحَيِّ وَالْمَيتِ
"The house of which remembrance of Allāh is made and the house in which Allāh is not remembered are like the living and the dead respectively."
So, the Prophet (May Allāh raise his rank and grant him peace) gave an example of the house where Allāh is remembered with the living and the house where Allah is not remembered with the house of the dead (i.e. graves).
When we combine the words of both Ḥadīths, we find that there is a similitude of those who remember Allāh (The Exalted) and fill their houses with the remembrance of Allāh (The Exalted) with living ones in houses filled with life, while the ones who do not remember Allāh and no remembrance is made in their houses with the dead in the houses of the dead, as if his chest is a grave for his heart with no real life in but an animalistic one.