Charles Ellicott Commentary Isaiah 58:5

"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? wilt thou call this a fast, and an acceptable day to Jehovah?" — Isaiah 58:5 (ASV)

A day for a man to afflict his soul. —The phrase comes from Leviticus 16:29, and describes the soul-sorrow which was the true ideal of fasting. In contrast with this, we have the picture, reminding us of Matthew 6:16, of the mechanical prostrations, which are like the waving of a bulrush in the breeze. The image suggests a new aspect of our Lord’s statement that the Baptist was not like a reed shaken by the wind (Matthew 11:7), namely, that his fasting was not outward and ceremonial, like that of the Pharisees.