Part of verse 28 of Surah Ar-Ra'd.
It describes the believers as those whose hearts find rest in the remembrance of Allah, and states plainly that it is in His remembrance that hearts find peace. The verse is often shared as a reminder of where real calm comes from.
- 13:28
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
(yaitu) orang-orang yang beriman dan hati mereka manjadi tenteram dengan mengingat Allah. Ingatlah, hanya dengan mengingati Allah-lah hati menjadi tenteram.