Matters of the heart.
Allah said in the Qur’an:Surah Ash-Shu'ara
يَوْمَ لَا يَنفَعُ مَالٌ وَلَا بَنُونَ ﴿٨٨﴾ إِلَّا مَنْ أَتَى اللَّهَ بِقَلْبٍ سَلِيمٍ ﴿٨٩﴾
“The Day when there will not benefit [anyone] wealth or children (88) “But only one who comes to Allah with a sound heart." (Surah Ash-Shu'ara 26: 88 -89)
`Ali bin Abi Talhah reported that Ibn `Abbas said about the Ayah,
إِنَّمَا الْمُؤْمِنُونَ الَّذِينَ إِذَا ذُكِرَ اللَّهُ وَجِلَتْ قُلُوبُهُمْ وَإِذَا تُلِيَتْ عَلَيْهِمْ آيَاتُهُ زَادَتْهُمْ إِيمَانًا وَعَلَىٰ رَبِّهِمْ يَتَوَكَّلُونَ ﴿٢﴾
The believers are only those who, when Allah is mentioned, their hearts become fearful, and when His verses are recited to them, it increases them in faith; and upon their Lord they rely Surah Al Anfal verse 2
"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﴾
Those who have believed and whose hearts are assured by the remembrance of Allah. Unquestionably, by the remembrance of Allah hearts are assured." (28)
Surah Rad verse 28