"[Those] who have believed and whose hearts are assured by the remembrance of Allah," meaning: they find tranquility. "Unquestionably, by the remembrance of Allah hearts are assured."
Muhammad said: "Ala" (Unquestionably) is a particle of alerting and commencement, and the hearts here refer to the hearts of the believers.
﴿الَّذِينَ آمَنُوا وَتَطْمَئِنُّ قُلُوبُهُمْ بِذِكْرِ الله﴾ أَيْ: تَسْكُنُ ﴿أَلا بِذِكْرِ اللَّهِ تطمئِن الْقُلُوب﴾.
قَالَ محمدٌ: (أَلا) حرف تَنْبِيه وَابْتِدَاء، والقلوب هَا هُنَا قُلُوب الْمُؤمنِينَ؛
