He traced it back to the Prophet ﷺ, who said, "Richness is not having abundant possessions. Rather, true richness is the richness of the soul."
حدثنا سفيان عن أبي الزناد عن عبد الرحمن الأعرج
يبلغ به النبي ﷺ ليس الغنى عن كثرة العرض ولكن الغنى غنى النفس
Its isnad is sahih. Muslim related it, 1:286, through Sufyan bin Uyayna, with this same isnad. Bukhari related it, 11:231-232, through another route from Abu Hurairah. Al-arad, with the letters ayn and ra both unpointed and with fatha, means the goods and possessions of this world.