لِيَجْزِيَهُمُ اللَّهُ أَحْسَنَ مَا عَمِلُوا وَيَزِيدَهُمْ مِنْ فَضْلِهِ وَاللَّهُ يَرْزُقُ مَنْ يَشَاءُ بِغَيْرِ حِسَابٍ ﴿۳۸﴾ وَالَّذِينَ كَفَرُوا أَعْمَالُهُمْ كَسَرَابٍ بِقِيعَةٍ يَحْسَبُهُ الظَّمْآنُ مَاءً حَتَّى إِذَا جَاءَهُ لَمْ يَجِدْهُ شَيْئًا وَوَجَدَ اللَّهَ عِنْدَهُ فَوَفَّاهُ حِسَابَهُ وَاللَّهُ سَرِيعُ الْحِسَابِ ﴿۳۹﴾

﴾38﴿ Liyajziyahumul laahu ahsana maa 'amiloo wa yazeedahum min fadlih; wal laahu yarzuqu mai yashaaa'u bighairi hisaab
﴾39﴿ Wallazeena kafarooo a'maaluhum kasaraabim biqee'atiny yahsabuhuz zamaanu maaa'an hattaaa izaa jaaa'ahoo lam yajid hu shai'anw wa wajadal laaha 'indahoo fa waffaahu hisaabah; wallaahu saree'ul hisaab

﴾38﴿ So that Allah may reward them according to the best of their deeds and increase them out of His bounty. And Allah provides for whom He wills without measure
﴾39﴿ And those who disbelieve—their deeds are like a mirage in a desert, which the thirsty one assumes to be water, until he comes to it and finds it to be nothing. But he finds Allah there, Who will settle his account in full, and Allah is swift in reckoning

[38] In this verse, there is glad tidings:
(أَحْسَنَ مَا عَمِلُوا) – Here, aḥsan (best) does not mean in contrast to ḥasan (good); rather, every deed that is done with sincerity and in accordance with the Sunnah is considered aḥsan—the best.
(وَيَزِيدَهُمْ مِنْ فَضْلِهِ) – This refers to the extra reward given for good deeds—greater than tenfold, and the exact amount is unknown.
It may also mean that beyond the equivalent return for their deeds, Allah grants them even more from His bounty.
(وَاللَّهُ يَرْزُقُ) – The provision referred to here is Paradise—whose quantity and measure are beyond calculation or limit.
[39] After presenting the example of the people of light, this verse mentions the example of the people of darkness.
The first example refers to the outwardly good deeds of a polytheist—such as prayer, charity, fasting, or acts of kindness to creation—that are mixed with shirk (polytheism).
The second example refers to the evil deeds, such as injustice, of a polytheist or disbeliever.
Or, according to a second explanation mentioned in Tafsīr Ibn Kathīr:
The first example is about those disbelievers and polytheists who call others to disbelief and shirk—meaning they are the leaders of kufr.
The second example refers to the ignorant polytheists who blindly imitate that shirk.
Explanation of the first example: (كَسَرَابٍ)—A mirage is what one sees at noon during summer in the open desert or field, which from a distance appears to be water but in reality is nothing.
Likewise, when one looks at a sandy desert in the sunlight, the glitter of the sand creates the illusion of water—commonly referred to in Arabic as "sarāb" (mirage), and in local terms as "gul-e-rīg" (false water in sand).
So a mirage is essentially the appearance of something that doesn’t exist.
(بِقِيعَةٍ)—This refers to a wide, flat plain.
(حَتَّى إِذَا جَاءَهُ لَمْ يَجِدْهُ)—These pronouns may refer to the mirage or to the deeds of disbelievers.
Meaning, when the person arrives at the “mirage,” he finds nothing; or when the disbeliever hopes for reward for his deeds, he finds it void.
(وَوَجَدَ اللَّهَ عِندَهُ)—According to the first interpretation, the full meaning is: "He finds nothing of value, and likewise, the disbeliever will find no reward for his actions, but instead will find Allah present with him."
According to the second interpretation, no additional wording is needed—the pronoun “with him” refers to the action itself, and it means the disbeliever will be confronted by Allah at the moment of reckoning for that deed.
Application of the parable:
Just as a thirsty person, thinking he sees water, runs toward the mirage and suffers hardship, only to realize it was in vain—likewise, the polytheist strives in doing seemingly good deeds, believing that they will bring him Paradise and reward.
But due to his shirk, he is deprived of Paradise, and his efforts go to waste.
Similarly, the leaders of disbelief, who gain worldly power and honor by calling to kufr and shirk, imagine that they will also have honor in the afterlife—but on the Day of Judgment, their assumption will prove false.