וַיַּעַן אַבְרָהָם וַיֹּאמַר הִנֵּה נָא הוֹאַלְתִּי לְדַבֵּר אֶל אֲדֹנָי וְאָנֹכִי עָפָר וָאֵפֶר – And Abraham answered and said, “Behold now I have commenced to speak to the Lord, although I am dust and ashes.”
The Gemara in Chulin 88b says that the reward for saying this was that his children would earn the Mitzvah of the ashes of the Para Adumah (Red Heifer) and dust of the Sotah (a woman brought to the Temple accused of adultery was forced to drink a concoction which had dust from the foot of the Altar in it).
Beyond the obvious yet superficial connection of dust/dust, ash/ash, how are Sotah and Para Adumah a relevant reward to Afar v’Efer?
The Dubner Maggid tells a story of an upstanding member of society who made a wedding for his son, and all the people and rabbis were invited. There was the top table for the family, and next to it, another for the rabbis. The greatest rabbi invited showed up, wished Mazal Tov, but felt unworthy of sitting in the prescence of the other rabbis, and quietly sat in the corner elsewhere. The host felt that the rabbi had not been accorded due respect, and he requested that the whole table of rabbis move to the table in the corner to join this great rabbi. He manipulated the context to make the supposedly unworthy corner into one worthy of having the great rabbi sit there.
The Dubner Maggid explains that this is precisely what Hashem did; He took what Avraham said, and changed the context from dust and ash with all their negative connotations, to dust and ash as Mitzvos, the essence of the Torah, the absolute opposite of dust and ash.
Another explanation was suggested by the Beis HaLevi: Dust of the earth has no past, but immense potential for the future; it can grow plant life, which itself is alive, can then sustain other life etc, whereas ash has no future whatsoever, but in the past was part of a living thing. Avraham meant he had no past, like earth, and and no future, like ash.
Says the Beis HaLevi, Hashem inverted this, by giving the Mitzvah of Sotah, which cleans the woman’s past, and Para Adumah which purifies the persons future.