At-Tawba 9:55

فَلاَ تُعْجِبْكَ أَمْوَلُهُمْ وَلاَ أَوْلَـدُهُمْ

(So let not their wealth nor their children amaze you...) In similar Ayat, Allah said,

وَلاَ تَمُدَّنَّ عَيْنَيْكَ إِلَى مَا مَتَّعْنَا بِهِ أَزْوَجاً مِّنْهُمْ زَهْرَةَ الْحَيَوةِ الدُّنْيَا لِنَفْتِنَهُمْ فِيهِ وَرِزْقُ رَبِّكَ خَيْرٌ وَأَبْقَى

(And strain not your eyes in longing for the things We have given for enjoyment to various groups of them, the splendor of the life of this world, that We may test them thereby. But the provision (good reward in the Hereafter) of your Lord is better and more lasting) 20:131, and,

أَيَحْسَبُونَ أَنَّمَا نُمِدُّهُمْ بِهِ مِن مَّالٍ وَبَنِينَ - نُسَارِعُ لَهُمْ فِى الْخَيْرَتِ بَل لاَّ يَشْعُرُونَ

(Do they think that in wealth and children with which We enlarge them. We hasten unto them with good things. Nay, but they perceive not.) 23:55-56. Allah said next,

إِنَّمَا يُرِيدُ اللَّهُ لِيُعَذِّبَهُمْ بِهَا فِي الْحَيَوةِ الدُّنْيَا

(in reality Allah's plan is to punish them with these things in the life of this world,) by taking the Zakah due on their money from them and spending it in Allah's cause, according to the meaning given by Al-Hasan Al-Basri. Allah's statement,

وَتَزْهَقَ أَنفُسُهُمْ وَهُمْ كَـفِرُونَ

(and that their souls shall depart while they are disbelievers) means, so that when Allah brings death to them, they will still be disbelievers, to make matters worse for them and the torment more severe. We seek refuge from such an end, which includes being led astray gradually by these things which they have.

Tafsir Ibn Kathir (abridged, English).