Source: Ṣaḥīḥ al-Bukhārī 6446
Allāh's Messenger ﷺ said: "Richness does not lie in many possessions; richness is richness of the soul."
Ibn Hajar's Explanation from Fath al-Bari
Ibn Baṭṭāl said:
The hadith means that copious wealth does not constitute real richness. For indeed, many people upon whom Allāh has heaped property are unsatisfied with what they have been given, and struggle to acquire more, and pay no attention to where it comes from; and such people are in reality impoverished because of their craving. Real richness is richness of the soul, which takes place when one finds a satisfying sufficiency in what one has been given, and does not covet more or struggle to seek it. That is the person who is genuinely rich.According to al-Qurtubi:
The meaning of the hadith is that richness that is useful, great or praiseworthy, is the richness of the soul. When a person's soul is detatched and independent, it loses its covetousness, and thus becomes exalted and splendid, and acquires more favour, detatchment, nobility, and praiseworthiness than the soul of the wealthy person, who reached his position by impoverishing his soul through his covetousness, which plunged him into base matters and ignoble practices through the meanness of his aspiration and avarice. The rich person thus finds himself condemned by many people, whose estimate of him is low, until he becomes the most despised and base of men.
Richness of the soul only comes about through richness of the heart, which must exist in absolute neediness of his Lord in all respects, realising that He is the Giver and Witholder, until he is satisfied with His decree and renders thanks for His blessings, and hurries back to Him to relieve his suffering. From this there develops the heart's absolute need of its Lord, and richness of soul which renders a believer independent of all that is not Him.