Musar על עבודה זרה 40:10

The greatest virtue of all the virtues that exist is the virtue of humility. This is why the greatest of all prophets is credited in the Torah with only this virtue. Rabbi Pinchas ben Yair lists a number of virtues, possession of each one is a stepping-stone to still higher virtues, the highest one being ענוה, humility. If the first human being who was the direct creation of G–d Himself (and could have displayed pride therefore) is described as having been formed of earth, this is a subtle hint that we must practice humility. Man was also called אדם because this word is an acronym for the words אפר, דם, מרה, as mentioned in tractate Sotah. The title אדם was bestowed only on the Jewish people, to the exclusion of the Gentile nations. We became heir to this title because of the humility of our great leaders.