Remembrance of God (dhikr) polishes what heedlessness has tarnished
This verse uses the same Arabic root for the heart's "rest" that later names the fully settled self at peace, tying the ordinary daily practice of remembrance directly to the psychological destination described elsewhere in the tradition.
Qur'an 13:28, from Surah al-Ra'd, makes a direct causal claim: hearts find rest specifically through the remembrance of God, using the same root that names the "self at peace," al-nafs al-mutma'inna, in Surah al-Fajr, so the two verses point toward the same destination by two different routes. Al-Ghazali, in Book 9 of the Ihya on recitation and remembrance, elaborates this into a practical claim about repetition: dhikr's effect is not primarily in the meaning grasped intellectually on a first pass, but in the cumulative, almost mechanical effect of returning attention to God again and again, the way repeated polishing removes tarnish a single pass would leave untouched. This connects directly to the mirror imagery al-Ghazali develops elsewhere in the Ihya: dhikr is the specific technique by which the rust of heedlessness gets worked off the heart, verse by verse, rather than removed in one dramatic effort.
Verily, in the remembrance of Allah do hearts find rest.
Qur'an; elaborated in Ihya Ulum al-Din, Book 9