'Sackcloth and Ashes' in the Bible

And in every province, wherever the king's commandment and his decree came, there was great mourning among the Jews, and fasting, and weeping, and wailing: many lay in sackcloth and ashes.

Is such the fast that I have chosen, a day for a man to afflict his soul, that he should bow down his head as a bulrush, and spread sackcloth and ashes under him? Wilt thou call this a fast, and a day acceptable to Jehovah?