Duties of the Heart
Among them, that which the people of Nineveh did, when they heard what the Creator decreed on them, as written: "So the people of Nineveh believed in G-d, and proclaimed a fast, and put on sackcloth, from the greatest of them even to the least of them" (Yona 3:5), and similarly what our fathers did in the days of Haman, as written: "And in every province, where 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" (Esther 4:3), and many like this in our books, when you investigate it, you will find it scattered frequently in the words of our ancestors.