Yeshayah 58:4-5

See, ye undergo a tzom for grievance and strife, and to strike with the fist of resha; ye shall not do a tzom as ye do today, to make your voice to be heard on marom. Is such as this a tzom that I have chosen? Merely for adam to afflict his nefesh? To bow down his rosh like a bulrush, and to spread sackcloth and efer (ashes) under him? Wilt thou call this a tzom, and a yom ratzon (a day of acceptance, an acceptable day) unto HASHEM?