Others said that it means the servant’s remembrance of his Lord in private, through which his heart finds rest and reassurance.
The second interpretation is that “the remembrance of Allah” here means the Quran: the Reminder He sent down to His Messenger, through which the hearts of the believers find reassurance. For the heart finds reassurance only through faith and certainty, and there is no way to attain faith and certainty except through the Quran. The heart’s rest and reassurance arise from its certainty, while its agitation and anxiety arise from its doubt. The Quran brings certainty and dispels doubts, conjectures, and delusions. Thus, the hearts of the believers find reassurance only through it. This is the preferred interpretation.
These same two interpretations also apply to the words of Allah Most High:
43:36 “Whoever turns away from the remembrance of the Most Merciful, We assign to him a devil who becomes his companion.”
The correct interpretation is that this means the Reminder He sent down to His Messenger—His Book. Whoever turns away from it, Allah assigns to him a devil who misleads him and bars him from the path, while he thinks that he is rightly guided.
These same two interpretations also apply to the words of Allah Most High:
20:124 “But whoever turns away from My remembrance will have a life of hardship, and We will raise him blind on the Day of Resurrection.”
The correct interpretation is that this means the Reminder He sent down to His Messenger—His Book. Hence, the one who turns away from it says:
20:125, 126 “My Lord, why have You raised me blind when I used to see?” He will say, “Thus Our signs came to you, but you forgot them; likewise, today you will be forgotten.”
As for interpreting this remembrance as swearing oaths, that is far removed from the intended meaning. The remembrance of Allah in an oath passes over the tongues of the truthful and the liars, the righteous and the wicked alike. The hearts of the believers find reassurance in a truthful person even if he does not swear an oath, whereas they find no reassurance in someone they distrust, even if he does swear an oath.
Allah has placed tranquillity in the hearts and souls of the believers. He has made those who possess tranquillity worthy of admiration and praise, and given them the glad tidings of entering Paradise. Blessed are they, and theirs is an excellent final abode.