﴿ ٱلَّذِينَ ءَامَنُوا۟ وَتَطْمَئِنُّ قُلُوبُهُم بِذِكْرِ ٱللَّهِ ۗ أَلَا بِذِكْرِ ٱللَّهِ تَطْمَئِنُّ ٱلْقُلُوبُ ﴾

“those who believe, and whose hearts find their rest in the remembrance of God - for, verily, in the remembrance of God [men's] hearts do find their rest-:”

those who believe and whose hearts are reassured find rest by God’s remembrance that is by His promise of reward. Verily by God’s remembrance are hearts reassured that is the hearts of the believers;