﴿ 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-:”

(Who have believed) in Muhammad (pbuh) and in the Qur'an, (and whose hearts have rest) their hearts are content and quiescent (in the remembrance of Allah) in the Qur'an; it is also said that this means: in swearing by Allah. (Verily in the remembrance of Allah) the Qur'an and swearing by Allah (do hearts find rest!) i.e. they are content and quiescent.