'Sackcloth and Ashes' in the Bible

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; and many lay in sackcloth and ashes.

Is such the fast that I have chosen? the day for a man to afflict his soul? Is it to bow down his head as a rush, and to spread sackcloth and ashes under him? Will you call this a fast, and an acceptable day to the LORD?

And I turned to the Lord God, to seek by prayer and petitions, with fasting and sackcloth and ashes.