Musar zu Könige I 10:27

And King Solomon, peace be upon him, essentially composed the book of Ecclesiastes in order that man put to his heart that the world is a vanity of vanities and [that] he only use it for the service of the Creator, may He be elevated. And he made his intention known in his introduction and his conclusion: For he opened and said, (Ecclesiastes 1:2), "Vanity of vanities, said Kohelet, vanity of vanities - everything is vanity." And our Rabbis, may their memory be blessed, said (Kohelet Rabbah 3:13), "If another man had said this, we would have said, 'Maybe he has not gathered two small coins in his entire days; therefore the world is considered like vanity in his eyes.' However it is appropriate for King Solomon - about whom it is written (I Kings 1:27), 'The king made silver as plentiful in Jerusalem as stones' -to say that the world is vanity of vanities." And he concluded his book and said (Ecclesiastes 12:13), "The sum of the matter, when all is said and done; revere God and observe His commandments, for this is all of man."

And Solomon was the richest of all men, as it is written, "And the king made silver to be in Jerusalem as stones" (I Kings 10:27). For him it was proper to say "Vanity of vanities, all is vanity" (Eccl. 1:2), and one ought not to busy himself with anything save for the reverence of Heaven. And thus did Moses, our teacher, peace be upon him, say: "And now, Israel, what doth the Lord thy God require of thee, but to fear the Lord thy God" (Deut. 10:12). And thus did David, the King, peace be upon him, say: "The fear of the Lord is the beginning of wisdom" (Ps. 111:10).