Matters of the heart.
God-Almighty in Holy Qur’an said:Surah Ash-Shu'ara
﴿ يَوْمَ لَا يَنفَعُ مَالٌ وَلَا بَنُونَ إِلَّا مَنْ أَتَى اللَّهَ بِقَلْبٍ سَلِيمٍ﴾
“The Day when there will not benefit [anyone] wealth or children (88) “ But only one who comes to Allah with a sound heart." ( 26: 88 -89)
Surah Al Anfal verse 2
`Ali bin Abi Talhah reported that Ibn `Abbas said about the Ayah,
﴿ إِنَّمَا ٱلۡمُؤۡمِنُونَ ٱلَّذِينَ إِذَا ذُكِرَ ٱللَّهُ وَجِلَتۡ قُلُوبُہُمۡ ﴾
(The believers are only those who, when Allah is mentioned, feel a fear in their hearts)
"None of Allah's remembrance enters the hearts of the hypocrites upon performing what He has ordained. They neither believe in any of Allah's Ayat nor trust (in Allah) nor pray if they are alone nor pay the Zakah due on their wealth. Allah stated that they are not believers. He then described the believers by saying,
﴿ إِنَّمَا ٱلۡمُؤۡمِنُونَ ٱلَّذِينَ إِذَا ذُكِرَ ٱللَّهُ وَجِلَتۡ قُلُوبُہُمۡ ﴾
(The believers are only those who, when Allah is mentioned, feel a fear in their hearts) and they perform what He has ordained,
﴿ وَإِذَا تُلِيَتۡ عَلَيۡہِمۡ ءَايَـٰتُهُ ۥ زَادَتۡہُمۡ إِيمَـٰنً۬ا ﴾
(and when His Ayat are recited unto them, they increase their faith) and conviction,
﴿ وَعَلَىٰ رَبِّهِمۡ يَتَوَكَّلُونَ ﴾
(and they put their trust in their Lord), having hope in none except Him. '' Mujahid commented on,
﴿ وَجِلَتۡ قُلُوبُهُمۡ ﴾
(their hearts Wajilat), "Their hearts become afraid and fearful.'' Similar was said by As-Suddi and several others. The quality of a true believer is that when Allah is mentioned, he feels a fear in his heart, and thus implements His orders and abstains from His prohibitions.
﴿ الَّذِينَ آمَنُوا وَتَطْمَئِنُّ قُلُوبُهُم بِذِكْرِ اللَّهِ ۗ أَلَا بِذِكْرِ اللَّهِ تَطْمَئِنُّ الْقُلُوبُ﴾
[ الرعد : 28]
سورة : الرعد - Ar-Ra‘d - الجزء : ( 13 ) - الصفحة: ( 252 )
Those who believe (in the Oneness of Allah - Islamic Monotheism), and whose hearts find rest in the remembrance of Allah, Verily, in the remembrance of Allah do hearts find rest
Surah Rad verse 28
﴿ ٱلَّذِينَ ءَامَنُواْ وَتَطۡمَٮِٕنُّ قُلُوبُهُم بِذِكۡرِ ٱللَّهِۗ ﴾
(Those who believed, and whose hearts find rest in the remembrance of Allah.) for their hearts find comfort on the side of Allah, become tranquil when He is remembered and pleased to have Him as their Protector and Supporter. So Allah said,
﴿ أَلَا بِذِڪۡرِ ٱللَّهِ تَطۡمَٮِٕنُّ ٱلۡقُلُوبُ ﴾
(Verily, in the remembrance of Allah do hearts find rest.) and surely, He is worthy of it.