Matthew 6:16

Isaiah 58:3-5 NIV
‘Why have we fasted,’ they say, ‘and you have not seen it? Why have we humbled ourselves, and you have not noticed?’ “Yet on the day of your fasting, you do as you please and exploit all your workers. Your fasting ends in quarreling and strife, and in striking each other with wicked fists. You cannot fast as you do today and expect your voice to be heard on high. Is this the kind of fast I have chosen, only a day for people to humble themselves? Is it only for bowing one’s head like a reed and for lying in sackcloth and ashes? Is that what you call a fast, a day acceptable to the Lord?

Matthew 9:14-15 NIV
Then John’s disciples came and asked him, “How is it that we and the Pharisees fast often, but your disciples do not fast?” Jesus answered, “How can the guests of the bridegroom mourn while he is with them? The time will come when the bridegroom will be taken from them; then they will fast.

Nehemiah 1:4 NIV
When I heard these things, I sat down and wept. For some days I mourned and fasted and prayed before the God of heaven.