Kohelet Rabbah 10:3/קוהלת רבה י׳:ג׳

1“Even when a fool walks on the way his heart is lacking; and he says to everyone that he is a fool” (Ecclesiastes 10:3).“Even when a fool walks on the way…” – a fool believes that all the people are fools like him, but he does not know that he is a fool and all the people are wise.