Tafsir of Ar-Rum 30:37

"Have they not seen..." (that is: have they not looked and witnessed) "that Allah extends the provision for whom He wills" (that is: extends it for him, Exalted is He) "and restricts it" (that is: straitens it for whom He wills to straiten it). This is either in consideration of two different people or in consideration of one person at two different times. The intent is to rebuke their exultation and despair in the states of prosperity and adversity; meaning: have they not seen that, so why do they not show gratitude or seek reward [from Allah] in both ease and hardship, as the believers do?

"Indeed in that" (the aforementioned, meaning the extension and its opposite, or all that has been mentioned) "are signs for a people who believe," so they may infer from them the perfection of His power and wisdom. How excellent is the one who said:

The hardship of the intelligent and the ease of the ignorant, Have guided you to a Perfect Wise One.

Al-Tibi said: The closing [of the verse] was His saying: "for a people who believe," as a notification that He, Exalted is He, does this by His pure will—Glory be to Him—and that wealth is not the result of a servant's actions and efforts, nor is poverty the result of his incapacity and failure. No one recognizes this except those who believe that this is the decree of the Almighty, the All-Knowing, as was said:

How many an intelligent man, with an understanding heart and complete reason, is poor and destitute; And how many an ignorant man possesses much wealth; that is the decree of the Almighty, the All-Knowing.