Musar על מלכים א 1:27

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."