The Sotah and The Parah revealed through Beresheet 18:27

"And the Lord said, 'If I find in Sodom fifty righteous men within the city, I will forgive the entire place for their sake.' And Abraham answered and said, 'Behold now I have commenced to speak to the Lord, although I am but dust and ash (עָפָר וָאֵפֶר). Perhaps the fifty righteous men will be missing five. Will You destroy the entire city because of five?' And He said, 'I will not destroy if I find there forty-five.'"

-- Sefer Beresheet 18:26-28

ALTHOUGH I AM BUT DUST AND ASH :

"As a result of this statement, Abraham's descendants merited the two mitzvot involving the ashes of the red cow, and the dust used for the sotah."

-- Baal HaTurim, Commentary on Beresheet 18:27

Moreover,

עפר = "dust"
=350
לשוטה = "for the sotah"
=350

ואפר = "and ash"
=287
בפרה = "regarding the cow"
=287

The "sotah" or suspected adultress, was forced to go through the ordeal described in Numbers 5:11-31.

The "red cow" is the Parah Adumah, whose ashes ashes are used for the purification ritual described in Numbers 19:1-22.

There is an entire tractate in the Mishnah called "Sotah" which deals with the laws of the suspected adultress. It is in the third order Nashim.

There is an entire tractate in the Mishnah called "Parah" which deals with the laws of the red cow. It is in the sixth order Taharot.

http://en.wikipedia.org/wiki/Ordeal_of_the_bitter_water

http://en.wikipedia.org/wiki/Red_heifer