Talmud – Mas. Ta’anith 16a
And where [on the head] does he put [the ashes]?-R. Isaac said: On the place of the phylacteries, as it is said, To appoint unto them that mourn in Zion, to give unto them a garland for ashes.1 (Mnemonic: open space, Ark, sackcloth, wood-ashes, dust, cemetery, Moriah.)
Why do they go out to the open space [of the city]?R. Hiyya b. Abba said: In order to express thereby [the idea], We have prayed in private, but we have not been answered; we will [therefore] humiliate ourselves in public. Resh Lakish said: We have exiled ourselves [from the House of God] may our exile atone for us. What is the difference between the two explanations? — The difference is when they move from one synagogue to another.2
And why do they take out the Ark to the open space of the city? — R. Joshua b. Levi said: In order to express thereby [the idea], We had a vessel which we kept hidden and now because of our sins it has been rendered common.
And why do they clothe themselves in sackcloth? — R. Hiyya b. Abba said: In order to express thereby [the idea], We consider ourselves animals3 [before God].
And why do they place wood-ashes upon the Ark? — R. Judah b. Pazzi said: As if to say, I will be with him in trouble.4 Resh Lakish said: [As if to say] In all their afflictions He was afflicted.5 R. Zera said: When I first saw the rabbis placing wood-ashes on the Ark my whole body shook. And why does everyone else put ashes on his head?- With regard to this there is a difference of opinion between R. Levi b. Hama and R. Hanina. One says: [To signify thereby], We are merely like ashes before Thee; and the other says: That [God] may remember for our sake the ashes of Isaac.6 What is the difference between them?
(6) This refers to the sacrifice of Isaac. Cf. Gen. 22.