Worldly wealth is a fleeting thing and is not an evidence of God's favour, and for good Muslims it is a danger signal
وَلَا تَمُدَّنَّ عَيْنَيْكَ (And never stretch your eyes - 20:13) The words are addressed to the Holy Prophet and are intended to provide guidance to his followers. They are told not to cast covetous eyes at the splendor and glitter of those who revel in the enjoyment of worldly pleasures, because all these things are fleeting and transient while the grace and blessings bestowed upon the Holy Prophet and through him upon his followers are everlasting and much more desirable than worldly luxuries.
People have always wondered at the wealth and prosperity of the infidels and evil doers despite their being loathsome and contemptible in the eyes of Allah, while pious and obedient Muslims spend their lives in poverty and destitution. Even the great and highly respected Sayyidna ` Umar al-Faruq ؓ was made aware of this glaring disparity one day when he entered the private quarter of the Holy Prophet ﷺ and saw him lying on a mat of rough reeds which left their marks on his body. Sayyidna ` Umar ؓ stood there and wept. Then he said, "0 Prophet of Allah! The kings of Persia and Byzantium live in comfort and luxury whereas you who are the chosen prophet of Allah and also his beloved live such a harsh life." To this the Holy Prophet ﷺ replied, "0 son of Khattab! Has it not dawned upon you yet that Allah has given to these people everything that is dear to them in this world, but they will have no share of the good things in the Hereafter, only punishment and pain?" This is the reason why the Holy Prophet ﷺ chose for himself a life which was free from the pomp and vanity of this wicked world, even though he had the means of accumulating all the comforts and luxuries of life. Whenever he received share of wealth even without physical toil or exertion, he immediately distributed it among the poor and the needy and kept nothing of it for himself. Ibn Abi Hatim has related, quoting Sayyidna Abu Said al-Khadri ؓ that the Holy Prophet ﷺ once said,
ان اخوف ما اخاف علیکم ما یفتح اللہ لکم من زھرۃ الدُّنا (ابن کثیر)
What I fear most about you is wealth and worldly splendor which will be arrayed before you. (Ibn Kathir)
In this Hadith the Holy Prophet ﷺ has foretold the Muslims about their conquests and the acquisition of vast territories which will bring them untold wealth and means of luxury. But there is no cause to feel satisfaction at these prospects; on the other hand there is every reason to fear such an eventuality because an excess of indulgence in luxuries may make people forget their duty to Allah.