Allah swears by saying:

﴿الْلَّيْلَإِذَا يَغْشَى﴾

(By the night as it envelops.) meaning, when it covers the creation with its darkness.

﴿وَالنَّهَارِ إِذَا تَجَلَّى ﴾

(By the day as it appears.) meaning, with its light and its radiance.

﴿وَمَا خَلَقَ الذَّكَرَ وَالاٍّنثَى ﴾

(By Him Who created male and female.) This is similar to Allah's saying,

﴿وَخَلَقْنَـكُمْ أَزْوَجاً ﴾

(And We have created you in pairs.) (78:8) It is also similar to saying,

﴿وَمِن كُلِّ شَىْءٍ خَلَقْنَا زَوْجَيْنِ﴾

(And of everything We have created pairs.) (51:49) And just as these things that are being sworn by are opposites, likewise that which this swearing is about are opposing things. This is why Allah says,

﴿إِنَّ سَعْيَكُمْ لَشَتَّى ﴾

(Certainly, your efforts and deeds are diverse. ) meaning, the actions of the servants that they have performed are also opposites and diverse. Therefore, there are those who do good and there are those who do evil. Allah then says,

﴿فَأَمَّا مَنْ أَعْطَى وَاتَّقَى ﴾

(As for him who gives and has Taqwa.) meaning, he gives what he has been commanded to give and he fears Allah in his affairs.

﴿وَصَدَّقَ بِالْحُسْنَى ﴾

(And believes in Al-Husna.) meaning, in the compensation for that. This was said by Qatadah. Khusayf said, "In the reward.'' Then Allah says,

﴿فَسَنُيَسِّرُهُ لِلْيُسْرَى ﴾

(We will make smooth for him the path of ease.) Ibn `Abbas said, "Meaning for goodness.'' Thus, Allah says,

﴿وَأَمَّا مَن بَخِلَ﴾

(But he who is greedy) meaning, with that which he has.

﴿وَاسْتَغْنَى﴾

(and thinks himself self-sufficient,) `Ikrimah reported that Ibn `Abbas said, "This means he is stingy with his wealth and considers himself to be in no need of his Lord, the Mighty and Majestic.'' This was recorded by Ibn Abi Hatim.

﴿وَكَذَّبَ بِالْحُسْنَى ﴾

(And denies Al-Husna.) meaning, the recompense in the abode of the Hereafter.

﴿فَسَنُيَسِّرُهُ لِلْعُسْرَى ﴾

(We will make smooth for him the path to evil.) meaning, the path of evil. This is as Allah says,

﴿وَنُقَلِّبُ أَفْئِدَتَهُمْ وَأَبْصَـرَهُمْ كَمَا لَمْ يُؤْمِنُواْ بِهِ أَوَّلَ مَرَّةٍ وَنَذَرُهُمْ فِى طُغْيَانِهِمْ يَعْمَهُونَ ﴾

(And We shall turn their hearts and their eyes away, as they refused to believe therein for the first time, and We shall leave them in their trespass to wander blindly.) (6:110) And there are many Ayat with this meaning, proving that Allah rewards those who intend good with success, while whoever intends evil is abandoned, and all of this is in accordance with a preordained decree. There are also many Hadiths that prove this.