ٱلَّذِينَ ءَامَنُوا۟ وَتَطْمَئِنُّ قُلُوبُهُم بِذِكْرِ ٱللَّهِ ۗ أَلَا بِذِكْرِ ٱللَّهِ تَطْمَئِنُّ ٱلْقُلُوبُ
Allazeena aamanoo wa tatma'innu quloobuhum bizikril laah; alaa bizikril laahi tatma'innul quloob
those who have faith and whose hearts find peace in the remembrance of God- truly it is in the remembrance of God that hearts find peace-
those who believe, and whose hearts find peace in the remembrance of God — truly, in the remembrance of God hearts find peace.
The verse answers a demand for a miracle made two lines earlier, and what it offers is an interior sign: the heart coming to rest. Ṭumaʾnīna is the settling of something after motion, used of ground that levels out and of a person who stops casting about. The second clause repeats the first with the words reordered and the possessive dropped, so hearts in general take the place of theirs.