Parashas Vayeira · Passuk 18:27
And Avraham answered and said, “Behold now, I have taken upon myself to speak to my Lord, though I am but dust and ashes.”
Torah Temimah points to Yerushalmi Taanis 2:1.
TT lemma: עפר ואפר.
English: “Dust and ashes.” The order of fasts: they take the ark out to the open place of the city and place ashes of a burnt offering upon it and upon the head of the Nasi and the head of the court. Why ashes of a burnt offering? In order to recall the merit of Avraham who said, “I am but dust and ashes.”
Derash on the verse (Yerushalmi) — not a formal named middah of the 13. A communal ritual is explained by reference to the patriarch’s words.
Ashes are placed on the ark and on the heads of the leaders during a public fast in order to recall the merit of Avraham, who called himself dust and ashes.