"I will make your progeny like the dust of the earth" (G-d's blessing to Abraham, Genesis 13:17). Just as the dust of the earth is from one end of the world to the other, so, too, will your children be scattered from one end of the world to the other. Just as dust is treaded upon by all, so, too, your children are destined to be treaded upon by the nations. Just as the dust of the earth is made fertile only with water, so, too, the people of Israel are blessed only in the merit of the Torah which is analogous to water. Just as dust erodes all metal utensils while it itself lasts forever, so is it...
Midrash Rabbah